Here is a thought
One reason some boats leave room between the head of the main and the halyard sheave is that if the main is very high aspect (tall hoist, short foot), and there is little or no masthead crane (so that the backstay attaches a few inches AFT of the top of the mast), then you will not have enough room for a reasonable roach on the mainsail if you go all the way up. So, one solution is to limit the hoist on the main (I am not crazy about this approach, though).
A PROPER measurement for a new mainsail (assuming you want a sail that really fits the boat), is to have the hoist, foot, tack setback and cut up, outhaul car cut up if any car is fitted, groove or slide sizes for the mast and boom unless loose footed sail is selected(these are the usual items), and THEN: back of mast near tack fitting to the BACKSTAY (straight back just parallel to the boom), and the length of the masthead crane (if you say 3", this means the backstay attaches 3" behind the masthead). With this info, the sailmaker can design a sail with the right amount of roach and be able to control whether or not the leech will hang up on the backstay. Most measurement forms will have these items called for, but many times they are omitted for a cruiser, but should not be-it is your $ after all.
Since we are talking 6", it is not going to make a huge diff. But whatever diff it does make will be positive. Make sure the sailmaker either measures the mast for bend (While taking all the other measurments) or understands the nature of the 27 rig (stiff). The "characteristics" are that the 27 is someewhat heavy by today's standards, benefits from the tall rig greatly in areas with winds often below about 10-15, and the lighter the typical breezes, the more a few inches will help.
I am out of the country this week, so a bit slow to repsond (Paris), but will be back over the coming weekend.
BTW- the best 27 guy around is Nigel B. from the website-and he is in your area. Whatever I tell you may be OK, but Nigel knows better, as he has truly optimized his 27 to the MAX-and a nice guy to boot!
Cheers,
S
