For a given CYA level, the loss of FC due to UV is a % of the residual, not a fixed amount. So the higher the FC residual, the higher the loss during the day. So yes, you need a higher SWG % setting for a higher FC residual. Water temperature seems to have a similar % effect on FC loss as well.

However, the FC demand due to organics could be a fixed amount assuming they are being added to the pool at a fixed rate (i.e. algae bloom). It takes so much FC to kill so much bacteria or algae. But the higher FC levels should kill it faster.

But I am surprised that you only lost 1.5 ppm with a 20 ppm residual. Even at 80 ppm CYA, normal FC loss would be 10-20% due to just UV. Did you have a cover on the pool at the time? Something else might have been going on at the same time.