I generally start with a 5/32" web at the muzzle and TRY to follow the center line of the bore to the breech; usually ending up at 1/8". However, given that my ramrod holes usually drift down, I frequently end up with more like 3/16" at the breech. In some early Germanic guns, you may need 3/16" or more at the breech in order to be able to get the front lock bolt to clear the ramrod. So, my answer is that it depends on a lot of things. Mainly, it depends on what you are building. Be guided by the originals.
As to the wood under the ramrod hole, I am satisfied with 1/16". It is hard for it to be more than that at the rear entry because of the lift on the rear thimble tang. If you managed to get the ramrod hole to run straight, then you might have 1/32" more at the breech. If not, then 1/16" is fine. Your guard front extension will break into the ramrod hole and that is Ok.