I don't believe you'll find a single answer. Elmer Keith used 20:1 and 16:1 lead:tin to develop the .44 Magnum. If the revolver's chambers are smooth and uniform and the bore is smooth and free of constrictions and is either uniform or tapers to a slightly tighter muzzle (half a thousandth to a thousandth tighter), but never the reverse, you will find it will shoot well with softer lead than guns not in such good condition for lead bullets.
Firelapping will produce smoothness. If the chambers are not perfectly uniform, you can get them reamed. You can have it done for a little under $40
here.
I find most revolvers and lever guns, especially as the alloy gets harder relative to the pressure of the load, will shoot a little better with bullets about 0.002" over groove diameter rather than the standard 0.001" over normally used for lead. Casting your own will let you experiment with that. The wider diameter removes some of the reliance on pressure to upset the bullet to obturate the bore, hence you can use greater hardness than your load can upset when using 0.001" over groove bullets.
Read the sticky on bullet hardness at the top of this forum. It will take your through some of the considerations. I also discovered I have an old Excel file for estimating minimum and maximum B.H.N. I don't even recall where I found the formulae? I'll have to review it. If I think it is reasonably reliable, I'll attach it or make it available for download. In the mean time, it needs your barrel length (I assume this is a revolver) to make the estimates, if you could post that? With the estimates, you can look up an alloy in the Lyman book or some other source of BHN v. alloy numbers. The LASC site has some, I believe? I'm sure the Cast Bullet Association forum does.
If in doubt, I would default to the standard Teracorp magnum alloy mix and experiment from there. That is 6% antimony and 2% tin and the difference being lead, IIRC, and the BHN is around 16? That should do fine in the range you are loading in, provided the bore isn't too rough.