Narrow it untill you feel it's in danger of becoming weak in a side to side direction ( assuming you have enough thickness to keep it non bending, or only bending a tad in the desired direction). Dont worry if you go too thin, you can always add a belly overlay to make it like a T section!
I've sometimes fancied maybe taking the tips to an 'I' section or a V section (pointed edge twards the back) for max stiffness with lightness.
No prob with nocks if you slap on a nice overlay, could even overlay the sides if it's an issue or do a small ELB style horn nock...
Damn you've got me wanting play....
So to sumarise, reduce it until if fails, then go back a tad
Del