Call Precision Sails and ask them for advice. We can't tell you here what they want before they design the sail. Every sailmaker does it a little differently.
If it's a headsail, you can't measure for it without the mast being up and guarantee it will fit.
If it's a mainsail, however, you can get adequate measurements even with the mast down, for certain boats. You usually need to have the mast up to measure for mast rake and prebend, but many old boats have no prebend and not more than 1 degree of rake. The Grampian is one of those boats if it has an original rig. It's a masthead rig, with negligible prebend and non-swept spreaders.
An experienced sailmaker can look at the rig characteristics and then tell you how much prebend and rake, if any, to tune into the mast. Precision Sails may be willing to do that for you (or not). Precision Sails can give you special instructions to get all the other measurements they need besides mast rake and prebend. if they're willing to take the extra time to hold your hand and double check your work. It's more work for them, and they may or may not be willing to do it
Talk to them about it, not us. They're the ones who will be designing your sail.
Judy B
Semi-retired sailmaker.
