Pyramyd Air has a useful tool for identifying airgun restrictions by state. I'm sure it isn't perfect but will give you information very quickly and easily that you should then check against the DNR of your state. I looked at South Carolina where I live, Georgia, North Carolina and Florida (surrounding states). None of these states require more than a 30 caliber but Florida does recommend a 35 caliber.
I was a PB hunter for decades before I ever shot more than a pump up pellet gun. 1,000 ft lbs is the normal rule of thumb but it's kind of a whole different world where the norm is for a 24 or larger caliber bullet leaving the muzzle at 2,500-3,500 fps. Our airguns are more like pistols with much heavier projectiles (for big game) going much slower. A 9mm (35 caliber) handgun would be unethical by most PB hunters for deer in my opinion. Too much chance of a lost wounded animal. If I ever hunt deer with an air rifle it will be at least a 35 caliber and probably a 45 caliber. I have no doubt a well placed brain shot with even my 50 fpe 25 caliber would kill a deer (and appears to be legal in my state) brain shots are frowned upon by many hunters since it is what poachers do. For a body shot my little 25 is obviously too small. A body shot is also where a 100-200 fpe 35 caliber is likely to have some trouble. I don't think you need 1000 fpe for a well placed shot with an airgun but trying for half that seems like a reasonable place to go.
So I would check the regulations where you want to hunt and possibly check with some more experienced hunters, especially any you may hunt with.
I also like to relate hole size to cross sectional area of the target animal. We are making holes with our airguns and how fast the target expires through blood loss logically is related to how big the hole is relative to their size. A .177 diameter hole in a squirrel damages .08% of the squirrels cross sectional area (side view). A 30 caliber hole in a deer damages .07%. A 35 caliber damages almost .1%. A 45 damages almost .16%. A 22 in a squirrel damages .13%. I think squirrels hit with a 22 drop a little faster than those hit with a 177. I use both and consider both ethical, however. But I care more about rapid expiration of a deer than a squirrel. So I think a 30 caliber is a bit light - smaller hole than a 177 in a squirrel on an equivalent basis. A little bigger than a 22 caliber in a squirrel seems about right to me (i.e. a 45).
