Actually you don't need to know any math at all.

Transfer your measurements to a piece of paper and measure the angles. Use a protractor. You can buy them for under $1 some places.

You can also simply attach the protractor to the neat frameworks shown in the pictures above. Attach a plumb bob (can make that for under $.05) and you've got as accurate an incidence meter as one you'd pay lots more than a dollar for and it cost you almost nothing.

You know, there is almost always more than one right way to do a job.