Hi moderngirl!
I measured the 75 cm belt strap from tip to tip (end to end) and it measures about 90 cm in length. This is just the leather strap, no buckle is attached.
From one end of the strap, measure 3 cm inwards. This is where the hole for the belt buckle will be. I threaded the hole with Hermes ribbon for illustrative purposes
From the other end of the belt strap, there are three holes for sizing. The first hole measures approximately 9.5 cm in from the end. The second hole measures approximately 12 cm in from the end. And the third hole is approximately 14.5 cm in from the end.
The hole on one end must match up with one of three holes at the other end of the belt strap to fit.
The easiest way to determine sizing is to cut either a long piece of paper (or smaller pieces taped together) or string down to 90 cm in length. Then either make holes spaced like the above pictures(one hold at one end, three holes at the opposite end) or mark the holes with marker. Wrap the paper or string around your waist and match up the holes and see how it feels.
Hope this helps!