Here's a pic or two of how I did it mate First run a line of tape at the distance described in the instructions - from memory 32mm up from the door line but check.

Start with the door as its the longest length. Tape the ruler at the bottom and align the strip on top by resting on it...



Then slide the ruler back and front to ensure alignment is the same...



This is how it works out - I used a meter long ruler here to achieve this