Your method of measuring lacks accuracy. If you want to have a go at doing it more clinically try this. It's a little skilled.

In lying place your palms on the top near outside ridges of your pelvis. People often reffer to this as the hips. Place your fingers on your outer leg and twist your whole leg in and out without bending at the knee. You should feel a bone going underneath your fingers. This is the greater trocanter of the thigh bone.if you can't find it move your leg outward slightly. Move your fingers upwards slowly untill you feel soft tissue and can't feel the greater trocanter of the thigh bone sliding under your fingers. Mark on the lateral leg the end of the greater trochaner with a pen.
Measure down to the lateral malleouls, the ankle bone on the outside. This should give an accurate measurement.

it's not something you can guess visually. There will be youtube videos on this.