Measuring up for new double glazed units – help please!

I want to replace two DG units at my house. I’ve got a chap to fit them but I want to measure myself to save some time rather than waiting for him to come round.
So where do I measure to? Is it to the join between the glazing bead and the frame less x mm or something different?

get them to do it. Seriously. Then if it’s wrong, it’s their fault and not yours.

Just measure between the lines where the bead hammers into and minus 10mm you need to get the thickness of the units though and without a glass thickness gauge or taking the units out you will have to guess either 24 or 28mm

get them to do it. Seriously. Then if it’s wrong, it’s their fault and not yours.

Do this. It might cost you a week, but it will be worth it if you do it yourself and get it wrong.

get them to do it. Seriously. Then if it’s wrong, it’s their fault and not yours

+100

Externally or internally beaded?? If internally pop beads out using two pallet knives and measure width height and thickness! Externally bit trickier due to the double sided tape applied to the inner pane and frame. This will need to be cut anyway when it comes to replace so do that and repeat as above. Or as above take 10 mm off!