Whew, I thought I overexamined things! ;-)

You're talking about some 23' foot monster there - but for the I20 / F18 here is how I go about it. Of note, I have an idler block in the center/rear of the trampoline to help take up slack when the spinnaker is snuffed. I typically mount this on a buji going through a grommet in the trampoline so if, for some reason, we need more sheet in a hurry, the bunji will give way and give us a little more.

That said, with the spinnaker hoisted I shoot for enough spinnaker sheet for the crew to be trapezed at about the rear beam and able to sheet the spinnaker as it just barely starts to pull on the aforementioned idler bunji. We usually start with the spin sheet a little on the long side and slowly snip away a foot or two after test sailing until it is right. That has worked out really well for us.