Hello,
Do you have any pictures of your 55 gallon?
Were you planning on mounting a tube bulb or did you want to use a Mercury vapor bulb with other lighting?
It would depend greatly on how you are planning on organizing the basking platforms & branches in the tank as to how far he should be from the lights also.
There are two types of tube bulbs you can choose from that require different distances & the Mercury vapor bulbs provide various UVB exposures along with heating.
If you want to use a tube bulb with a basking bulb, I would recommend either the Reptisun 10 T5 High Output tube bulb along with a bright white Halogen flood light or a Par 38 basking light. A 55 gallon you could probably get away with 1 75 watt or a 100 watt bulb for basking, at 8-12 inches away. You can mount the T5 bulbs on top of a screen & they are strong enough to go through the screen & still provide good UVB at 8-12 inches. The placement would depend also on whether or not you plan to use a reflector in the tube light fixture, too.
There are many other options too, with other lights. Let us know more details so we can help with planning for you.
Tracie