In the beginning, I left a little extra slack on both sides because I was still unsure which way I wanted everything to be arranged. You know, whether I would have the buckle smack-middle or as suggested by some off-midline to the right to avoid crotch strap and busiest area.
Nowadays, when I know how I have my things arranged I only leave enough extra to allow for some adjustment (winter/summer undies with drysuit, singles/doubles rig). It's a pain to have too much especially coming from right but you also gotta watch it not cutting it too short for your thickest exposure protection. On the left one can have some more slack because it stores neatly backwards onto itself and that way you can move the location of the buckle two ways if you later choose so.
I usually leave about 5 inches extra on right side (beyond closed buckle), so that the slack tucks conveniently into my knife webbing loop on left side of the buckle. This way there is enough to handle it even with dry gloves. Very minimal amount makes it harder for me to grab. If you know where the buckle will be there isn't need for more than an inch beyond the buckle on the left side.
These measurements are with your thickest garment needs. Coming from Ontario you probably dive 7mm or drysuit, so if you measure things for that you will be pretty safe leaving fairly minimal amount on the left (except if you want to experiment with buckle) and some on the right to adjust for possible add-on protection. Just remember you might want it looser than you first thought, and it's always easier to cut more off than put it back on
And when you start putting the webbing on the plate, do not cut it until it is in place and you have measured it on yourself with hardware on! It's very easy to go overboard with the cutting, and not take into account for example the extra you need for weaving back onto the buckle. I know, I have experience on that one!