Go to a table and bend over it. How far down do you want to bend. Then think of what you want to do under it, if you want to roll a cart then that has a height, that’ll be your floor + 2inches. Height is totally subjective. I didn’t want to bend over and I wanted to use it as a work surface, but not an assembly surface. So its like 64" or so tall (I’m 6’3) and I have a husky cart, a wood trolly, and a garbage can under it at the moment. I use the hybrid tracks to mount things like a router table. YMMV. Just back into it based on what you can envision yourself wanting to do and kinesthetically how you want to work on the surface. Worst case, you redo it at some point. 
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