Distilling down everything I've read it seems to be: whatever is comfortable for you.
If you get into engraving you're going to be sitting at that bench working for
hours at a time so if something's a little too high or low you
will feel it. Likewise if your setup requires you to be in any sort of contortion you will (eventually) feel that too.
Since you don't have a scope yet you'll want the top of your vise up quite a bit higher so you can work with the least amount of stooping over. You may even want to work standing up (since you've got a Palm Graver and don't have to worry about working a foot pedal).
When you get your scope the advice I've retained is: Set your chair so you're sitting comfortably and ergonomically correctly - including having your foot on the foot pedal if you have one.
Now set your scope so the eyepieces are right in front of you so there's no strain in looking through them for as long as you like.
Now bring your vise up to where it needs to be to see it.
In theory you're now all properly positioned. In realilty you'll probably tweak things a few times before you find the perfect setup.
Good luck and be sure to show us some of your work!
