You measure as they show you in the cheat sheets online.
Additionally, measure beam at the mast
and then max beam.
Next measure spreaders length and how high they are from the base. Most cats have spreaders (much) shorter than max beam and this may be important for the sailmaker
, depending on what sail you get.
Have a drawing of the rig for the sailmaker too - maybe there is one already for your boat somewhere online. If not, draw it yourself.
Take pictures, they help sailmakers.
My tip: consider having a sailmaker over to your boat for measuring. This is actually very common for custom and semi custom rigs.
If you want one sail to cover the scenario with the tack at the center as well as the tack at the windward hull
, you measure for a kite that fits the central mounting point, not the beam bow point.
You measure 100% dimensions. The extra luff length of the G2 will be decided by te sailmaker.
(My own preference is for minimal extra length - as I get very upset sailing in light airs whenever our kites collapse and get caught on our bow rails).
Tip: some makers have G2 styled sails sold off the shelf ... these can be easily priced 2/3 of a custom one.