Just working out how much rigid insulation I’ll be needing for the roof/walls of my little Outer Hebrides project. Looks like it will be in the region of 59 sheets (2.400 x 1.200mm). Is there a rule of thumb or way to estimate how much 100mm silver aluminium tape I will need to get?

Could I suggest either that you don't use foil tape at all - use air-tightness tape - or that you buy one roll of foil tape from a number of different suppliers? I have had very mixed experience with the adhesive on foil tape. Much of what I have bought in the past I would not bet on for 10 years, let alone 60 (assumption for life of air-tightness tape).Re the length of tape I tend to draw it out if I don't just seriously over-buy, but I think if you allow 4.8m (1.2 + 1.2 +2.4) per board and then measure the whole perimeter of the room(s) and add it all up and then add 10% you should be about right.

As above from redbeard, foil tape is, generally useless. Some of mine fell off overnight. I did buy some "cold weather" foil tape. That was quite good.

Airtightness tape is much better, though im sure as its become more widespread, theres crap versions of that too. The stuff i used as bonkers sticky!

I've used mountains of foil tape in the floor and ceilings and found it totally fine. Was left up on the ceiling for a year before I boarded and it was still stuck solid. As for estimating how much, I just bought a load of it and when I needed more just went and got it, no method in my madness.

Just any old stuff from Screwfix worked for me, some has backing paper some doesn't. When you get in the zone and your technique down it just flies on.