I want to apply to Northwestern, University of Michigan, Yale, Princeton, and Cornell. The first two don’t require subject tests but will consider them if you send them to show academic strengths. I’m planning on taking Bio M, Math 2, and Chemistry. What would be a good target score on the subject tests for these schools? Thanks!
800
Mid 700+, but may depend on your expressed interests, grades in those subjects and how well you did on your SAT1/ACT to give context to the SAT2 scores.
If you intend on being a STEM major, anything under say 790 for Math 2 is probably a fail.
You should aim for nearly perfect or perfect scores. Subject tests are a lot easier than their AP equivalents, and in my experience schools like the ones you’ve mentioned expect a lot. However, I know people who scored a 700 on SAT Physics and were accepted to UCLA Engineering, so I think that these tests are something that can only help you, and doing badly won’t hinder your chances that much 
Unlike the SAT, which everyone takes, the SAT2s are taken by a self-selected group of people .
- People who are applying to top colleges that require the SAT 2
- People who think that that subject is a strength
So for Math2, a 750 is only a 60th percentile. For Math1, it is a 92 percentile. For regular SAT Math, it is 97 percentile.
So for Math2, you are comparing yourself to other top mathy kids, not the general population. So an 800 is at the 79 percentile. So the answer is 800.
https://secure-media.collegeboard.org/sat/pdf/sat-subject-tests-percentile-ranks.pdf