The headstay is very easy to set up, there are just a few points to keep in mind:
Too tight and it adds stress to the stem and mast fittings
Too loose and the headstay will sag
A tighter headstay is better for sailing to windward
A looser headstay is better for sailing down wind
You need to headstay to be longer than the luff of your headsail
With synthetic standing rigging, I like to set up the headstay and backstay at the same time. I set the rake with a slight aft bias and then tension the headstay and backstay until it feels right.
Once the right amount of tension has been achieved, simply tie the Shroud Frapping Knot on the headstay lashings and tiddy up the ends.
You will quickly know if the headstay is too loose, as it will sag and the luff of the headsail will fall to leeward.
If it is too tight, the headsail will never sag. This may sound ideal, but the sails are designed to work with a bit of sag. If the headstay is so tight that it won't budge, it will place extreme stresses on all of the fittings and the sail will not perform as well as it could.
If you are not setting up the headstay and backstay at the same time, set the length needed on the headstay and when the backstay is set up, the headstay will subsequently be tensioned. The headstay and backstay pull against each other via the masthead, you should use this fact to your advantage when setting up your rigging.
I like to take advantage of an adjustable backstay to fine tune the headstay tension while under sail. If we are going to windward all day, I'll tighten the backstay adjuster. If we are running or broad reaching, I'll ease the backstay adjuster and let the headstay sag a bit, filling the sail with more air as it sags out into the wind. More about the adjustable backstay in a later post.