Generally you go up .001" for cast lead. Some folks will say to "slug" your bore to fine tune.
Molds generally drop AT their stated size or *slightly* larger.
One other issue is the polygon barrel. Net wisdom says it's a no-no, but there are more than a few folks that claim to shoot cast in their polygon barrels. My suspicion is *they* are right and the polygon legend had more to do with the idiot who blew up a Glock shooting unlubed boolits, and the fact that 9mm and .40 are pretty high pressure and require a *hard* boolit.
If the alloy is too soft, the pressure of the charge overcomes the tensile strength of the alloy, pushes past and "gas cuts" the boolit (think acetylene torch cut principle) leading up the barrel. If too hard, the boolit doesn't take the shape of the bore, the lube blows out, and gas cutting takes place.
People think the lube is for lubrication, and to some extent, I'm sure that's true. But it's most important function is as a liquid piston ring to hold back the gasses to prevent gas cutting. It only has to hold long enough for the boolit to take flight. If it blows out then you get gas cutting.
9mm runs about 33,000 psi, so a bhn of 26 is about right. 26 is pretty difficult to attain with any lead/tin alloy as I understand it. But I got 26 with water dropped wheel weights on my first try. Wheel weights are pretty ideal alloy though- other than being a little short on tin.
Oh, and don't be surprised when your 9mm boolits come out .358"- they do double duty for .38 special, .357 magnum etc. which run larger than 9mm para/luger. I wouldn't even bother testing first with that mold- just get the sizing die.
S'cuse my soapbox, but one last note: The Lee modern reloading book has quite a bit of info about using cast boolits. I imagine similar information is not unlikely to be in other handloading books. My Speer book didn't have much, but they manufacture bullets, so I can see why they might not want to share such info.