Not sure if this has been asked before, but
If the supplier doesn't have a recomendation on this, as a rule of thumb how tight or how big a gap should the bottom of the baseboard be from touching the surface of the floor installed?

As tight as possible. You are refering to fitting the baseboards to the walls and floors, correct? The floors should be gapped about 1/2" from the walls. Always nail baseboards and baseshoe/quarter round to the walls and not the floors.