You need to come up with a logical way to derive a rating but none of this is an exact science and would be open to some debate - I'm assuming the rig comes from a Gcat 5.7?

G-cat 5.7 (without spin): 72.7
18HT (with spin - 2up): 59.6

Modifier for boat normally with spinnaker sailing without one: 1.020

18HT (woutOUT spin - 2up): 59.6*1.020 = 60.80

now is where we have to get creative; the 18HT w/o spin is at 60.8 and the Gcat at 72.1. In this scenario, I would estimate that the rig probably has about 80% to do with boat speed and the hull (w/ modern daggerboards) and boards have about 20% effect. So I would take the difference between the two ratings, and put the rating on the boat it 80% toward the G-cat rating over the 2-up spinless 18HT.

Gcat - 18HT(no spin) = 72.1 - 60.8 = 11.3
80% of the difference = 11.3 * 80% = 9.04
18HTGC = 60.8 + 9.04 = 69.84

69.84 is where I would start.