How should I calculate 1099 taxes for my side-gig?

I have a small side-gig in addition to my main full time salaried job. My full time job is W2, but for my side-gig I am a 1099 contractor. I'm trying to figure out how I should calculate my taxes so I can set aside a chunk of each payment from my side-gig rather than be hit with a larger bill each quarter for estimated taxes.

The problem is, it's a little tough to estimate exactly how much I'll make from this thing this year. It could turn into a really big income supplement, or just a few small paychecks and then fizzle out, depending on how things go.

Is this math right, at least until I get out of my current tax bracket? Taxes owed = 1099 Income x (15.3% (self-employment) + 12% (current federal income tax bracket) + 5.75% (current state income tax bracket))

Edit: Also, for calculating when I've crossed the threshold into the next tax bracket, I can effectively add the standard deduction to my income, right? i.e. As I am married filing jointly, I won't enter the 22% tax bracket until my income passes 106,950? (Since I'll have to be at $81,051 after subtracting the deduction.)

Edit again: Oh! And if I can deduct 50% of my SE taxes, does that essentially further increase the threshold before I cross into the next tax bracket?