Is there a general rule of thumb on the width of the top of the comb nose? I realize it's a curve but generally speaking the upper portion. I generally end up at about half the width of the grip cap but not by design, just end up there. I think I could improve and speed things up if I laid out lines on each side of the centerline and used that as a guide for my plane. I'm pretty much learn by trial and error but have gained the most insight from people here so any advice is appreciated.

Interesting question.

I have an old German 8mm sporter where the comb nose is knife thin. It's a slender stock all around and that width seems to fit and function fine.

I don't think I would want that on a big kicker, though.

Keep in mind I'm no stock maker.
That said it comes down to the lines of the rifle. As in what looks good. There are no hard and fast rules out side of the basic dimensions of the stock. i.e. Length of pull, drop @ heal, drop at toe, drop @ comb, cast on/off, pitch of butt plate. After that is what fits the over all design

I typically use the stocks over all shape and the receiver as a guide. Slender stock, slender grip etcetera. On a Springfield or spade tang Winchester I will leave it a bit more rounded as this seems to be a better transition from receiver to stock on the top.