The viewing window for on this tape measure

Find a stick the same length as your arm. Hold your arm out straight and level with the stick pointing straight up (90° to your outstretched arm). Walk forward/backward until the tip of the stick coincides with the top of the tree. Your feet are now at approximately the same distance from the tree as it is high. (For a more precise approximation, back up by the additional distance of the height of your arm above the ground.) The relationship is true only if the tree is significantly taller than you are, and the ground is relatively level. Time-tested logger method. Simple.

Forget it!!!! Trying to measure an untrue room for prefabrication of parts is nearly impossible. Your carpenter will have to come in, establish a center line in both axis and divide the differences on the edges. This application is no different than doing a suspended ceiling, and must be done on site. There will be a bit of adjustment in all directions, I'm sure.