Hey, and welcome back!
I haven't built a straight Hill style longbow, so I can't help you out on stack thickness. But I can help you out with a couple other questions.
First, the glass will be parallel. Your wood laminations will either be all tapered, or a mix of tapered and parallels. Your stack thickness will be measured at the butt end of the lams. You will want some tapered lams to get the tiller right. I think with a Hill style, you want around a .006 taper, but wait for someone who's actually built one of those to chime in.
And one extra little bit of advise, zebra is really porous. I've used it in a few bows under clear glass. Make sure you use smooth-on and make sure it's warm, so it runs a little nicer when you glue everything together. It's easy to get bubbles under the glass if it's thicker. I've had to remake a couple pairs of limbs cuz I'm a little slow sometimes