It is very easy to oversheet a squaretop mainsail, you want to sail a squaretop with noticeable twist in the leech. You'll go faster that way. However, if you still feel that the top of the mast is still to soft (not stiff enough) than the best and easiest solution is to move the hound fitting higher up the mast. The is a 3rd order dependency so only slight movements may produce large effects. The longer section between the hound fitting and mast base can then be stabilized again by the spreaders. And if you feel this is insufficient then longer spreaders arms will correct this. In principle and within reason you can fune-tune the mast to the new mainsail design. It is not a unchangable given, unless the class rules limit were everything may go.

Example : stiffening up the top of the mast. Say a mast is 8 mtr. long and the hound fitting is at 5.75 mtr. The top length is then 8-5.75 = 2.25 mtr. and assume it is too soft in the top in this setup. You want the top to be 20 % stiffer then before. 3rd order dependency => 120 % = 1.2 = (2.25)^3/(new top)^3 therefor (new top) = (old top)/(3rd order root (1.2)) and new hound height = 8 mtr - (new top)

(new top) = (old top)/(3rd order root (1.2))
(new top) = 2.25 mtr/1.0627 = 2.1173 mtr. = say 2.12 mtr. or 130 mm shorter

And new hound height = 8 mtr - (new top) = 8 - 2.12 = 5.88 mtr.

20 % extra stiffness then before is quite alot.

So you can see; 130 mm movement of the hound fitting can be quite alot.

Hope this helps

Wouter