You can not. Variations in bullet weight (also case, if the brass is mixed) can easily be larger than variations in the charge.

Math is fundamental. But you're doing it wrong. Whether or not it can be overcharged doesn't matter, because your gun doesn't need a double charge to blow up. Likewise, you don't actually care whether the case has enough useful capacity to hold enough powder to blow up, because compressing the charge could cause a pressure spike.

But what the hey, let's calculate it anyways.

First, you need to know the volumetric density of Alliant Blue Dot. Multiply the charge weight in grains by the VMD, and you get the colume in cubic centimeters. You're going to use this in reverse.

Note that the actual VMD of powders can vary by as much as 16% in canister powders. I can't remember where the hell I read that.

http://leeprecision.com/cgi-data/instruct/VMD.pdf

Next you need to know the case capacity of the .45 Colt. At a length of 1.285", we'll just go ahead and say the capacity is 2.696cc. Note that this is the overall case capacity, not the useful case capacity. We would need to know how much case volume was taken up by the bullet to know that.

So we plug that into our VMD formula: 2.696cc = .08650 * X. Then we solve for X, dividing 2.696cc by .08650.

We get 31.16 grains! Even if we lobbed off half to account for the volume occupied by the bullet (and there is no way the bullet is occupying half of the case), we still get 15 grains. Considering that the max I'm seeing on the Alliant website is under 13 grains (too lazy to go downstairs and see what my books say), I'd suggest that you can't mathematically show the ammunition to be safe.