The distance really depends on the type of bulb you're getting. Did you order a T5 with a fixture that has a reflector? In that case they're pretty strong so you could either mount them 12-14'' above the basking area inside the enclosure, or you could mount it on top but bring the basking area a bit closer (elevate stones, put in taller branches etc) so that it's around 10'' or so to compensate for what gets lost to a screen top.
Are your current bulbs the ReptiSun compact fluorescents that screw into a standard socket? If so, do you know which wattage they are? If they are 26w they should be OK for now until your new one arrives. If it's the 13w version, those aren't really strong enough to provide adequate UVB. Probably not a huge deal in the short term, but I would supplement with some time outside for natural sunlight if you can in the case that the shipment of the new light is delayed for more than a week or so.