Soo I lost my 4.0 last week to AP Calc BC. Background: I’m a sophomore right now who has had a perfect 4.0 GPA. Last week after final exams, I lost my 4.0 by 0.04%, and my teacher won’t curve it 
How bad does an A- math look if I want to go into a CS/Engineering field at a top 20?
You’re overreacting. Pretty much nobody has a 4.0 unweighted, and there are few sophomores taking an AP math. It’s still an A, so calm down, you’re not going to get rejected because of it.
With BC in 10th, presumably you’re planning on going on to classes like multi variable, linear algebra, etc.,
Do well in those (or whatever advanced math you go on to) get an A next semester in BC, do well on the BC AP and no matter how highly selective the colleges are that you end up applying to, they won’t care at all abut an A- in first semester BC.
I agree with what many others have said, but what score is an A- in you school? If it is a 90 or better many colleges will consider it an A any way. Many school systems don’t use the +/- grading scale.