Is there an ideal trail?

I talked with some one when I first started to put my hack rig together, he said that I could install a '80 triple tree in a '81 honda cb900 custom which would reduce the trail. Well I got a triple tree from an '80 and I'm ready to install, but I found that I got ripped off on the used TT, the steering stem is bent. So being that I have a few machine tools and know how to use them, I cut the weld off and knocked out the bent stem. I'll make a new stem tomorrow. But I'm wondering if I should add a bit more rake by boring the hole for the stem at an angle, effectively adding more rake to the TT.
By installing the '80TT I calculate that I will change the trail from 4.88" to 3.17". If I add 2 more degrees to the TT I calculate the trail to be 1.87". So assuming I know what I'm doing with the machining and calculating would 1.87" be too little trail? Is there an ideal trail?
Is there a good way to actually measure trail? I've tried but eyeballing the neck axis to the ground looks to be just a guess.