How does one estimate how much taxes you may owe if you are applying for an extension?

Military retired pension W2 is incorrect.  Only recourse is to put in a "ticket" that they say they will have an answer to in 30 days.  That answer puts us at March 22nd, then will have to wait for the corrected W2.  We want to do a tax extension, but have no idea how to even estimate how much taxes we MIGHT owe this year