Use some string........builders line is best....(it has to have a small amount of stretch, not to much)

i take it that your birds mouth will end flush to the base of the wall plate..... if so tie the string to the ridge beam, bringing the knot to the bottom edge you wish to start from and then( small) nail it to the bottom of the wall plate...

this will not only give you your length of run, but from it you can also obtain the angle for the "perp"/vertical ridge beam cut, and for the "perp"/vertical cut on the birds mouth.........

you could use two battens also.....

but the old ways are best!!

hope this helps!!

Thanks folks. I used string to set the ridge but I think I'll try using a batten to measure up for the rafters.

Ended up cutting two rafters (one for each pitch as slightly different) and using these as templates for all the others. All went up ok.

Will soon have to figure out how to get the membrane up there. Think I'll have to make something that hooks over the ridge to support the roll while I move the ladders.