Hi, Captain Vimes.
Don't worry about YBS0502's set-up, it is fine.
He wrote: "THE BALLAST Sylvania 49136 QTP2X54T5 HO UNV PSN HT is the one i went with". I checked this out, it is listed on Amazon:
http://www.amazon.com/Sylvania-49136-QTP2X54T5-Fluorescent-Ballast/dp/B002OMCHVC
It is a 2 Lamp Universal Voltage PROStart Programmed Rapid Start Quicktronic
Electronic Fluorescent Sylvania Ballast for 54 watt T5 HO Fluorescent Sylvania Light Bulbs
That is fine; if you read what he wrote again, I am sure what he meant was that he was customising his old fixture (ie., the actual fluorescent tube holder) by re-wiring his new T5 ballast into it in place of his T8 magnetic one, and swapping T8 pin connectors for T5 ones.
This is not a suitable procedure for anyone with no electrical experience to do, but it should work just fine....
Your lovely UK twin T5 Arcadia electronic ballast with endcaps is not yet available in the USA. Hopefully, it soon will be.... along with their single T5 ballast for folks who only want one tube.
You are absolutely correct about the suitable lengths for the different size tanks, as there should always be a UV
gradient, with the tube over the basking zone and extending some way down the tank towards the cool end, but never actually reaching it - there must be a UVB-free shady area there, as you say.
Best wishes,
Frances