Hi Colin,
Hope you are well.
I would advise to watch our measuring guides as this information is all in this video I have attached below for you:
We do advise the below when your measuring for the headrail:
– Measure in 3 places for both the width and the drop, and always use the smallest measurements
– Remember the drop provided will be a combination of the headrail and slats
– Remember, to consider the bunching option of the blind when fitting by opening windows or doors.
If you want to send some images to our email feel free and we can also look at these for you.
Please let me know if you need any further help in the meantime as I am happy to help you.