Leave the saw at the 90 degree mark and make a fence to sit on the table that allows you to cut on either side. Like a sliding table for your table saw, but instead of moving the stock and the slide, you move the saw blade.
I've not done this with a chop saw; I did do it with my radial arm saw and it worked fine.
Even if your angle is off a bit, it will work. If one side is 44 degrees, then the other side will automatically be 46 degrees and make a perfect 90 degree angle.
I used an aluminum framing square for my "fence". I drilled and screwed it to the baseboard. Then I sliced through it with my first cut.
After that I cut each corner with on miter on one side and the other miter on the other side of the blade. With the complimentary angles it will always yield a tight miter.