How much of a difference is there between velocity measured at the muzzle (as shown in reloading manuals) versus the muzzle velocity taken from a chronograph 10 ft from the muzzle? Is there a formula that can calculate the difference?

It can be calculated by using your ballistic coeficient. My old Accuchron cronograph had a chart for some average screen distances from the muzzle but in any case it's not much.

Rest easy and use 15 fps. It does not matter that much now you can see so if that's off by such and such a few digits it does not make much difference.

A easier solution is to leave the chrono home and add 100 fps to all loads. While this is not scientific it's easy and fun.

Do not put much faith in velocities printed in loading manuals.They are not using your gun or your components so the chances of your specific gun producing the exact velocities printed in the manuals is very slim.Even with the same brand and model of rifle and components ,differences in individual barrels and powder lots can easily result in differences of well in excess of 100fps.

I agree with Savage99. Instead of calculating it, I just add 15 FPS and have done with it!!

logger,
There's an old method of computing this by multiplying the distance from the muzzle to the center of the screens by 6.4 fps and adding that too your actual instrument reading to convert to muzzle velocities. This is only an approximation since it doesnot consider bullet shape. I think it will get you within about 1 fps. That's close enough for me. Good luck.