Have some Reba SL's with 92mm of the stanchions showing. I know they can be 80mm or 100mm travel.

Would I be right in thinking that they are set for 80mm travel? (I ordered 100mm travel)

Edit: ignore that daft question

Let all the air out and measure how much stanchion is showing. I find that it should be the relevant travel + 10mm

Could be 80mm (the amount of stantion isnt the amount of travel) but if they are air the negative spring could be to firm, pulling the fork down, meaning they could be 100mm.

Zip tie around stanchion.Let out all air,and lift fork to full extension.Measure travel of fork at full extension,ie from zip tie to seal,which when lifted,will tell you what travel they are set at.
As suggested negative air may pull the fork down a bit.
Reinflate to desired pressure adding + first,then-.Try them at equal pressures first,and check travel.