What's the rule of thumb on when it's most worthwhile using GUF vouchers to upgrade?

I've flown quite a bit this year and am forming a small collection - have one GUF2, two GUF1s and an Amex companion voucher. Business trip coming up to IAD, which I'm considering doing ex-DUB and using a GUF1 on to fly First, but noticed it's only 36,000 Avios to upgrade and not sure what the best value option is.

When given that choice, what should I consider? I assume by best combo is the companion + GUF2 if I can find a way to use that, but what about the GUF1s? I guess I should relax about getting absolute best value, but thought I'd check first so I don't burn a voucher then kick myself.

I do apologise as this must have been asked but I can't find it.