I would think the maximum fill level would depend a bit on filtration. If the filter is strong and creates a lot of surface agitation, you would need to leave a bit more space than if it's quiet on the surface.
I did once fill a tank WITH a rim 'till it was over the edge on the inside of the rim. But it wicked up between the acrylic gadget I use to hold the lights up and the glass. So it dripped outside. Now I fill to just below that edge on the inside.
I'd think an inch of space is plenty, half an inch might be fine too, provided the tank is level and nothing you put on the rim can wick water up. If it can you'll find out quickly, the dripping can make quite a mess in a short time