First, one needs to know the definition of a Brinnell #: It is literally the pressure (Kg/mm-sq) at which point the alloy begins to deform.there is no explanation on the site about what that number (1422 x .90) represents.
Take a look here:
http://castboolits.gunloads.com/show...=1#post1886949
The "1422" number comes from converting kg/mm-sq to lbs/in-sq = 1422.332639
The 90% comes from getting real close to (but not over) yield pressure where the alloy just lets go on you.
Be that as it may, however, I've found using a 1.3 multiplier (rather than 0.9) gets me in the ballpark -- adjust from there.
I STRONGLY NOTE: that using a softer alloy is more often than not the best route to go