Measuring frame angles can be tricky, especially with today's molded frames, where seattube angle measurements might be effectively altered by any offset at the bb shell, or by shaping/tapering or even curvature of the seattube.
And headtube angle measurements are made impossible when the headtube is tapered.

Even with traditional frames made from straight, round tubing, subtle sloping of the toptube must be accounted for.
I measure frame angles from one side of the bike, then point the bike in the other direction with the tires resting on the same two spots on the ground, and then measure from the other side of the bike while keeping my same static position.
Averaging each of the resulting pairs of angle measurements will then cancel out any slope of the ground and/or any slope or the bike's toptube!

Lastly, electronic angle-finders usually have some "hysteresis error" to their measurement, such that measurements taken after the instrument is tilted/rotated in one direction from the "zero" reference (usually the toptube) are different from measurements taken after the instrument is rotated in the opposite direction from the "zero" reference. So again, pairs of each of now four measurements need to be taken, rotating the instrument in both directions from zero, meaning that a total of eight measurements will need to be taken in order for the seattube and headtube angles to be accurately documented. I do this all the time, and at least it goes pretty quickly by now.

At least with most modern bikes, geometry data can usually be found online, though it takes some effort to be sure to find the actual geometry for the exact model year.