Using a ruler does not seem to be the way to go from what I have read on the forum. Apparently, there is precious little difference in the wheelpants’ height above the floor when the tire is very underinflated. There have been several threads on this issue in the past few months. You should be able to find them using the search function, or if there is someone else more skilled than I out there that can put a link up for you, that would be better.
The difficulty in checking the tire pressures stems from two factors. First, the valve stem is not oriented at a convenient angle and second, you have to access it through a hole that is only about 1 1/2" in diameter.
Anyway, I use a ball foot type air chuck and a straight in type chuck with the shrader valve on the end of a 6" long solid tube. I have to use the ball foot type for the mains and the straight in type for the nose (or vice versa). Others have said they were able to get a single gizmo at an RV store that worked. It is also advisable to pick up some tire paint at an automotive store so you can place a mark on the tire at the bottom when the valve lines up with the access hole. This makes the whole process much simpler when you are doing it by yourself. I would pick up one of those portable, electric air compressors for about $60-70 available at just about anyplace that sells automotive accessories.
Finally, you will need to either put a little tape over the plugs for the access holes or use some tape around their circumference where they snap in to the pants. If you don’t, they will eventually depart the plane when you fly. I understand they are just ‘panel plugs’ available at your local hardware store, but I have checked a couple where I live and I can’t find any large enough. Cirrus sent us several before we figured out the ‘solution’. We have been using white electrical tape to hold ours in.
Hope this helps. Good luck with the new plane.