Skeater, I measure it for length only. Usually 4". But I usually cut it a little longer initially, add maybe 1/4", cuz sometimes I lose a little length as I stretch it around. If I dont lose any while fitting it up, I can always trim it later.
I cut it a bit long in the front too at first, where the stitching will go. Then work at pulling it around and down into the curves, and get it fitting good, in general... removing it to trim the top or bottom edges that are obviously getting too long. It can 'grow' as you pull and fit it down into the contours and you may have to trim it several times.
The last part cut is the front, where the stitching goes. I just wrap it around where it needs to be, overlap the two pieces in the front, and mark them at the center of the front of the handle, and cut that excess off.
At this point it is all fitting good, the top and bottom edges are straight, and the ends that will be stitched are straight down the front of the handle, either just touching if the leather is a type that won't stretch much more, or separated by 1/16" or a hair more if I think the leather is stretchy enough that they will pull together when stitching.
At this time, I skive the last 1/4" or so of the top and bottom edges of the leather, on the underside, so when it's layed on the bow, it 'rolls into' or fades into the bow, showing no edge to the leather, but it isn't necessary of course.
Once everything is fit and trimmed, I remove it, and spray the inside of the leather with the adhesive, excluding the last 1/4" or so where it will be stitched. It's unnecessary there and the glue just gets on the needles making them harder to push through the leather. With the adhesive on the back of the leather, it's then carefully put in place where it's supposed to be, paying particular attention that it goes properly down into the contoured area, and then stitched down the front.
I do NOT prepunch holes. I just use two big needles, start at the top, and sew my way down to the bottom, careful to maintain even thread spacing by eye. Done. Any questions, fire away.