Do you have access to a bow with a similar draw weight that you plan to use at your draw length?

Then simply measure how far you are drawing an arrow to the back of the riser, you can relay that info to the builder. You can use a plain arrow and a bread tie to mark the position.
The problem with going through all this now is that your draw length will most likely evolve if you are new to this and your muscles strengthen and things start to stretch out a bit.
Don't be surprised if you order this bow at a certain weight at a certain draw and you wind up shooting it a few lbs heavier at a longer draw length at some point in the future.