This is a bad idea. Short-term memory is the first to go under pressure. And the SAT is exquisitely designed to short-circuit your short-term memory.
Write out *every step* of the math solution.
Here’s an example of one of my old SAT tests.
You can see I write out all the steps.
Can I do some in my head? Yes – but sooner or later I will mess up and I am UNWILLING to give up even and SINGLE SAT point due to careless error (or any other reason.)
So – show *all* your work and increase your SAT score!