There are several here that use t8s. I think most use the 48" versions however. They really don't put out enough light for Oncidiums, but Phals and Paphs are great candidates. 18" is probably too far away. Why not spring for a decent light meter like
Light Meters, 840006,840020,21,22 (I use the 840020 Lux/Foot-Candle meter) and you can tell for sure. Other than that you are just guessing at the output for several reasons. What about reflector? What about condition of bulbs? What about distance? There are just to many factors to allow guesses. If you are serious about growing any sort of plants (orchids included) under lights, you need two things - a good light meter (like the above) and a good pH meter to check on pH of the solutions you are delivering. This is a different discussion, discussed here many times, do a search.
Welcome aboard and I wish you success in your growing. Ask away if you have other questions.