Not another "what length underquilt" I hear you groan. Well this is a little different.

What I'm actually interested in is: is there a "rule of thumb" about length when it comes to full length underquilts? Something along the lines of "If you are X cm/inches tall then add Y cm/inches"? Obviously there's a maximum length imposed by the gather of the hammock, no point in insulating unused space but what about minimum/optimum?

Part of the reason for asking is that here in the UK getting Stateside goods can be an exercise in frustration and extra cost: a Hammockgear Incubator that sells for $379 will have $30 for postage (no problem with this, it's got to be paid somehow), then when it gets to the UK we pay 8% duty on that $309 then 20% VAT (sales tax) on that and then the carrier can charge $10-$20 "release fee" so that $379 becomes $410. Not cheap. There were some UK makers but they seem to have fallen by the wayside so there's just Cumulus based in Poland this side of the pond.

Their quilts are long. The full length quilts are 215cm (84") or 220cm (89") while their 3/4 length quilts are 180cm (71"). The latter is only 2" shorter than the short version of the Incubator.

I'm 180cm tall. Could I use the 3/4 as a full length? This is assuming that the head end is just to shoulder/neck and I use a pillow or jacket under my head. That would potentially give me 20cm (8") or so extra at the foot end. I do a lot of bikepacking so minimising bulk is quite important. Method to my madness or madness to my method?

I've a couple of Cumulus quilts and have no problem with their quality. I know that they do custom work on the quilts so I'm assuming that they would be prepared to do so on their underquilts if an extra 10cm or so would be beneficial but I haven't asked them yet.