Well this is pretty much impossible to answer I think... unless someone has any experience with T. Macrogonus. Even then, if reports of mescaline content are quite varied as you seem to suggest, it's impossible to tell without trying some of your sample. I'd certainly want to err on the side of caution. Maybe you could try a low dose of 10 grams which would leave you with around 100 grams, and see what that does. If it gives you light effects, then it's probably safe to assume that around 30 grams would be a nice dose. If it gives you nothing, then maybe go for more.
As for P. Harmala to potentiate, it certainly works. But given the almost complete lack of knowing the potency of this cactus, I'd pass on that until you know more. It does potentiate to at least twice the potency, possibly up to three times. It also makes it a bit more stimulating in my experience.
I'm not sure what I'd do in your situation... I'd probably try 10 grams, then move up to 30. Then if that's close, you'd have enough to have a proper trip. If it's still not even close, you could try 25-30 with the syrian rue. I've potentiated cactus twice with syrian rue and enjoyed the experiences and didn't feel like it was dangerous, but I was working with cactus of a relatively known potency. And a pretty low dose (15 grams dried peruvian torch... or maybe it was san pedro).