Playfields will have differing angles of measurements for several reasons stated previously.
I’ve compared the phone app you are talking about with a digital inclinometer and the app is inconsistent. The inclinometer is only as accurate as it’s calibrated for. I calibrate my inclinometer with a small torpedo level against a leveled flat surface of a wall.
Keep in mind that when taking measurements on the playfield, if the inclinometer is not parallel with the top, bottom or sides, you won’t get an accurate reading. If it’s a steep angled playfield the inclinometer will tend to slide or move from where you placed it so you have to have a way to have the inclinometer not move.
After all that measuring, you still might have to tweak the level of the PF to make the ball travel or not travel like you want. You may have to set the machine up to lean to one side to avoid STDM drains. That is one aspect of where “dialing” a machine comes in.
Setting a machines level is just the beginning or a starting point for ball travel. A lot of it may require tweaking after a level and pitch baseline has been established.
You can do it, it’s not hard once you understand the above and don’t get stuck on the idea that a PF has to or only can be set to a measuring device.