7500 mm from the top of the mainbeam. So it dependent on how tall your maststep is. As such you can't measure it from the top of the mast.

But for a 7750 mm long luff you want the gate at this F16 height or indeed you'll be sailing with a spi luff that is far to loose.

Just lay the boat on its side and measure the spi gate height that way from the top of the mainbeam.

Wouter