Your measurements are only as good as the tools you’re using. If you count on accurate measurements, perhaps it’s time to put your tools to the test.
Framing squares can throw you off if they’re not spot-on. You can test yours in a few minutes, and you might even be able to correct the error.
Start by drawing a long straight line using an accurate straightedge. Sit the blade of the square tight to the line, and draw another line along its tongue.
Now, flip the square over, so the blade continues to sit on the baseline, and the outside corner of the square sits tightly in the intersection of the line you’ve drawn.
If the tongue of the square sits along the second line you drew, you have a square square. If it doesn’t, draw another line along the tongue.
If the square is less than 90 degrees, you can try aligning it with your center punch. Lay the square flat on a hard surface, such as an anvil. Place the punch just to one side of the inside corner and give it a good whack. Do the same on the other side of the inside corner. This expands the metal, and increases the angle. To decrease the angle, do the same by the outside angle.
Test using your line until you have it right. More tests next time.