FWIW land depth is not the same on all barrels either so there is really not a math formula you can use. Buy a stoney gauge and a dummy shell for each caliber you load in. You will most likely have different readings for each different type bullet you use.

Like the rest said, most likely they'll all be different and as such just learn to accept and live with it.

BTW just touching is often the worst place you can seat bullets too. A very good rule of thumb to follow is to either stay at least 10th off the rifling or at least 10th engaged(or more in either case). Just touching may well be that with one bullet but the next may jump etc... they are not made perfect and this can play heck with accuracy and pressures.

Jeff