PIR board wastage - rule of thumb?

[Digmixfill]

Is there a rule of thumb percentage add on for tongue and groove pir cavity board ordering?

I've had a quick look around for wastage figures for t&g pir in cavity installations but haven't turned up anything yet.

[Declan52]

 You don't waste that much as you use the bit left over from a cut to start another row. Like you do with wooden flooring so the joints are all staggered. It will be small triangle bits that you will be left with when you come to the peaks.

[Digmixfill]

Perfik!