Hi Jacob! I see you’re doing a great work in learning Persian! Your questions are getting more professional every time! :D To be honest I did some research to be able to answer your question thoroughly. First of all there’s a small detail about the verb “فکر کردن” that I think you’d better know: In fact فکر کردن in Farsi means intellection, reasoning or believing. But a few centuries ago when people started translating European languages to Farsi, they started using فکر کردن as a translation of “thinking” which sometimes has the meaning of guessing and other things of this sort that you know, but before then we used the word “گُمان کردن” in such contexts(subjunctive contexts!). And thus everyone forgot that فکر کردن and گمان کردن are completely different in meaning. However nowadays we use فکر کردن with both meanings and it’s considered correct, although some professionals do not agree. Having all the above in mind, I give a rule for deciding whether or not to use فکر کردن with subjunctive ( please note that this is for written Farsi):

3.It depends on the situation. فکر میکنم انها درست می گویند. یا فکر میکنم حق با انها هست.(است.)

The most accurate English translation for "فکر نمی کنم درست باشد", in my opinion, is "I don't think it would be true". In fact, باشد (باشه) here is making it hypothetical, while است is more actual and solid. Therefore, فکر نمی کنم درست است is, in a way, ungrammatical. As for the affirmative, you could use both, but you should bear in mind that فکر می کنم درست باشد is a bit more doubtful, since باشد makes it hypothetical and imaginary.

2.Both sentenses are correct. فکر میکنم درست است. فکر میکنم درست باشد.

1.this translation is better and we usually use this: فکر نمیکنم درست باشد.