Reddit mentions of Digital Angle Finder Protractor 7 Inch Stainless Steel Angle Finder Ruler 200mm with Zeroing and Locking Function, Digital LCD Display, Coin Battery Included for Woodworking, Construction, Repairing

You have 2 angles to measure, as you've discovered. One will translate to the angle of the blade in relation to the fence, the other to the angle of the blade in relation to the ground. My verbiage may not be correct, but you'll see that there's two places to change the angle of your miter saw. To find these, use an angle finder. A cheap one is ok, something like this will do just fine. https://www.amazon.com/Protractor-Stainless-Woodworking-Construction-Repairing/dp/B074GNVMCJ?ref_=fsclp_pl_dp_6

Measure the angle, divide it by two, and adjust your saw to that number.

On the example shown, the top miter would only require one adjustment to compensate for the horizontal change as the vertical angle remains consistent or appears to do so. The lower miter would also only require one angles be accounted for as this time the vertical angle changes but not horizontal. The process is no different than if you were mitering the baseboards on the stairs below.