After viewing the Mountain Man website and playing with the program a bit, I am impressed. However, how do you know what parameters to plug in? What do you ask for? I realize its for advanced casters, but outside of buying several moulds in each caliber, how would you develop a "favorite"?

If you could narrow down what the design weight you are after, ie pistol, rifle, caliber, weight, I might be able to offer some pointers.

The first tip would be to design something along the lines of existing designs. I'll measure existing bullets I have with a caliper, note measurements, and go from there to get in the ballpark, and then tweak it on the software.

I like driving bands to be between .080" and .100" wide, larger for larger calibers. Meplats of 70-80% of bore dia work well. Nose length should be chosen so that the round will fit in your chamber. For a light for caliber bullet, I like the keep as much bullet as possible in the case. For heavy for caliber bullets, I like as much of the bullet weight in the nose as possible. For bottlenecked rifle rounds, keep mindful of the neck length so the gas check stays in the neck.

No reason to consider it for advanced casters. Just for people with $60 to burn. ;-)
Invest in a chamber cast. Browings has cerrosafe for not much.
With that, and an idea of weight/velocity you're looking for, you'll be far along.
With most modern chambers the lead is the limit, not the weight; I like to find the heaviest bullet you can run to 2200 and go from there. Roger has a 300g going 2k out of a tiny (41mm?) 308 case.
If it's a pistol, most find a balance between weight and speed.
A 454 might love a 270g, and a 45 Colt a 240g. The ACP does great with 230's. A FN typically does better terminally, but RN's feed better in autos.
And so on.
What are you shooting, and what are you shooting at?

I am shooting lots of different calibers, and in most of them, I am interested in working up a light practice loads for the kids' deer rifles. They shoot 6.5x57, .270win., .280rem., and 30-06. For my guns, I have started with the RCBS 215 gr RNGC and it has turned in pleasant accuracy with the first load- 21 gr. Unique in a .358Norma Mag. I see myself becoming very interested in making up a cast bullet hunting load for my .375 Winchester, since top speeds exist in cast bullet range, i.e. 2100-2200 fps.
Heavy bullets are fine, and so I guess a chamber cast would be helpful. The Winchester Big Bore 94 seems to be limited in C.O.L. more than leade, so now what? I appreciate any and all input. Finally, what about diameter? Is .376 best? .377? Puzzling....

You will need to slug your bore to determine your bullet diameter. I like the egg fishing sinkers. Clean the bore well and lube it. Lube up the sinker and tap it in the muzzle end, then tap or push it through with a soft rod. Use a good micrometer for measuring, not a caliper. Most like a bullet 0.001 to 0.002 over bore diameter. You can alway size down a little if needed. It's also nice to be able to shoot bullets as cast. Remember to specify your alloy as subtle change can affect diameter. If you plan ahead and get a mold a tad big you have lots of options open. Too small and your SOL.

I haven't personally shot a 375 but have heard that it is very picky about COL and that it has a long oversize freebore.

Even if your goal is a low-recoil plinking load a heavy-for-caliber bullet would be my first choice because the longer bearing length seems to help compensate for the gun's imperfections.

You may already know this but if your bullet casts to big you may size it down enough to remove most of your bullet lube grooves. So like "BBore" said just a tad big is ok.

If I only knew how much a tad, a pinch, and a smidgen measured I would be happy.

A tad is just over a frog hair thick. A smidgeon is a fine blonde one. A red one is way too much.

In regards to your 375 bullet. Dan at MM has a forum so you can ask questions.

http://www.mountainmolds.com/phpBB2/index.php

You'll want to go a thou. or two over your slugged bore size with your alloy specified. Using your chamber casting you need to size the bullet nose to set in the throat. The front band, above the cannelure, can be adjusted to be just off of the rifling. The nose length, nose diameter, and front band are all a balancing act to get it right. If you have a good casting and accurate dimensions, Dan will be real helpful on letting you get it right.

There's also a guy on the Bearthooth and MarlinOwners forum by the name of RanchDog. He's done a bit of work with the 375 and recently got into casting. Might be a good contact. Marshall, at Beartooth Bullets, may also be able to answer some questions. His archived articles have some good stuff on bullet design. They're dealing with the 444, but the thought process transfers.

3584Elk, for plinking and practice with the kids' rifles, I'd say just get a Lee mould off the shelf. No need to specialize. Better yet, bulk jacket bullets. Just too easy.
For the 375, definitely slug the barrel, but I'd still recommend a chamber cast if you're going to be picky about the bullet. You're right about the COL; it's kinda like a mini-45/70 with that little nose, but if you like a wide meplat for heaviest bullet, you're going to want to know what that leade looks like.
There are already at least a couple good 250g moulds out there. Check them out.

Thanks fella's very informative.

I have used lots of cheap jacketed bullets in the kid's rifles, but the fun of casting and the lack of copper fouling mean a little bit more to me.

I guess I better do a chamber casting on the Winchester, as well as slugging the bore.

I would say if you are going to cast for the smaller bores start at 30 and work down, and tackle the 6.5 last. I don't if the 6.5X57 has the same fast twist as the 6.5X55, but fast twist barrels are the most difficult to get accuracy with cast, and can be a major exercise in frustration, even at mild levels.