Checking Dimensional Details:
All the necessary measurements are taken inside the premises with just a measuring tape and a water level. 
Every dimension should be taken at least in three points:
H : opening height (distance from floor to opening top);
B : opening width (distance between left and right side of the opening);
h : headroom (distance between opening top and ceiling) min 150 mm;
b1/ b2 — left/right side room clearance (distance between a corresponding opening edge and obstacle) min 120 mm;
L :  backroom.
When measuring Н and В the largest figure is taken for final dimension.
When measuring h, b1, b2 and L the smallest figure is taken for final dimension.