I'm using Unique powder and a batch of 220 grain flat point Berry's bullets. The Berry's website says to use the powder manufacturer's data, but Alliant has no data for the 220 grain.

They have data for 225 grain: 11 grains Unique.

I want light charges because the revolvers are older, collectible Smith and Wessons. The cases are 44 mag.

My favorite bullet for these guns is the 240 grain SWC plated with 7 grains Unique. That gives an indication of what kind of rounds/recoil I like in these guns.

I'm thinking 7.5 grains/8 grains Unique for the 220 plated.

Thoughts?

Should be a fairly mild load.Ive used between 7 and 10 gr with a 240 gr cast bullet

The powder charge for 225gr bullets should work just fine for 225gr bullets provided they are the same type (cast, plated, jacketed, etc.). It is pretty much always safe to use the next lighter bullet with any given powder charge as long as the bullet profiles are the same. In the larger pistol cartridges even the bullet profile pretty much becomes irrelevant.