I've played around doing the same thing with 7.62x54R, changing the FMJ to soft points. There are several assumptions that can be made: If you're using the same diameter and weight of bullet, your pressures should be the same. Lighter bullet, lower pressure. Heavy bullet, more pressure. These are only assumptions, so beware.
Because I have no way to measure the pressure, I make comparisons with a crony with original factory/surplus ammo to the loading data in my manuals. In a general sense, the calculated weight of your bullet and its speed should define its pressure. The dangerous variable is you don't know for certain what is the type of powder in the original load and can only make estimates by comparisons to powders with similar results. If you have a known powder with known data, then its easy to load from the bottom with low pressures.
Load light and don't go over max. It works with your own powder, but pay attention to your data and measurements. You should be able to calculate your increments of speed adjacent to your increments of powder if you load down from your starting point to begin your data.
If you don't have a chronograph, stay completely away from any of it. Actually, I don't recommend it at all. My only goal was to pull the bullet and change from FMJ to SP for hunting. I try to keep the speed and pressure the same, and it does work.
