I think Mustachianism doesn't necessarily mean always trying to be as rational as possible, but it does include recognising your psychological quirks to twist them for your benefit. Treating part of your income as "disposable" because of where it comes from just seems like a way to rationalise spending that you otherwise wouldn't do. Dollars are dollars - they don't come with "save me" or "waste me" labels.
That doesn't necessarily mean you shouldn't spend it how you do - just think about it and perhaps you'll come to the conclusion that that really is the best use of that money for you. (In that case, the psychology can work in your favour - you're using your money effectively, but also if those stocks do poorly then you'll cut that spending as a direct consequence.) Or maybe you'll come to the opposite conclusion, and then you'll start using your money more effectively because you want to.
In answer to your question, no, I don't have a set amount of money which I allow myself to spend suboptimally. (Well, I guess I could say the amount is zero, but being human, I don't achieve it, it's just an aim.)
