It's very easy to figure out, but the fact is, you must figure it out. There isn't anything magic to BHN, just a formula.
Here's a definition of Brinell Hardness Number (BHN) from businessdictionary.com:
http://www.businessdictionary.com/definition/Brinell-hardness-BH-test.html
It's a long definition but basically BHN is pressure measured in Kilograms/millimeter^2
To convert from Kg/mm to PSI, you can multiply by 1422
(Note to penpal - this is PSI, not CUP)
PSI=BHN (or Kg/mm) x 1422
90% of that is optimum for cast bullets, so the final formula to use is:
PSI=BHN*1422*.9
All you have to do is figure out your load pressure, then select the correct hardness. Or if you have a bunch of bullets sitting around to use, you can create a load of the proper pressure to use.
If it was only the weight of the bullet, the accuracy wouldn't be effected with the change in BHN like it is. The only way accuracy could be effected is because of the obturation, i.e. the pressure acting on a given hardness.
When I use the word "accuracy" here, I'm really saying "grouping" as, all else being equal, the heavier bullet will show more drop. Grouping should remain the same, however.
I have proven this to my satisfaction using my 303 Savage, .309" lead bullets of a known BHN and different loads of pistol powder to increase or decrease the pressure. Small changes of quick powder such in a rifle changes pressure without doing much for velocity.
People using Unique with their 30-30 plinking loads will agree with this in a heartbeat. At some point up the scale, bullets start flying erratic, but some weight just below that is a sweet spot that's extremely accurate.