This is always a tough question. When I first started cutting lines I would buy a really cheap FWF, cut it and re-cut it if needed to get the feel for the line with the rod. Then I would upgrade to a Mastery or something and the balance was way off again! I did learn a lesson, but it wasn't the one I had hoped.
As my circle of hybrid head buddies increased, I got more recipes from people who had made the plunge with their lines already and that was a huge help.
IMHO - What it really boils down to is three things, which I am sure you already know...
a) grain weight and distribution for the rod
b) complementary to the tips you want to fish
c) line control and presentation
a) If it doesn't work for the rod, it's wrong. That's a no brainer. If it's too light, the rod won't load; if the head's too heavy and short, the cast becomes a risky chuck and duck. yada yada.
b) I think we all have favorite tips, or at least we start to accumulate tips with certain pools or water levels. If you are fishing through a situation and "THE" tip doesn't cast well on a particular line configuration, you aren't going to be happy. Therefore the back half of the head should accomodate the tips you want to fish.
b2) On the other hand, the tips can also accomodate the head. Just as a quick switch of a tip can change the way a line casts drastically - there may be a workable tip for the line as it has been cut. I tend to favor the tip because it governs the presentation over specific pools and thus I'd rather have a line that accomodates the tip than the opposite.
c) Line control and presentation on top of AND below the surface - these are the reasons we ask such questions, figure out good hybrid mixes, and don't just use teeny lines and/or full sink lines in this application. We want to be able to cast a hingeless head, mend the line, and reduce the amount of flourescent chartruese fly line whiplashing through a gin-clear winter pool.
So it stands to reason that a real short back end with a long tip would be fine in a down and across situation but it would reduce the line control advantages of mending long casts, etc. In the half and half situation (about 15 feet of a 30 foot WF body) half the "heavy stuff" floats and is mendable, half is not. If for no other reason, this is at least balanced for line control.
My experience has been that cutting too little is a waste of time and creates a useless floating tip. I suspect that cutting too much sucks too. There's probably a fairly large slop factor provided you are willing to adjust the tips to match the head portion.
In any case... good luck and please (everybody) log the cuts you've made and the outcome for future inquiries of this nature!