Do they still make the 8ft fittings? I haven't seen them for ages. When you say capacitor are you talking about the starter or the PFC capacitor? I'd suggest around a 14uF would be close enough, there probably is a formula for working it out but rule of thumb is roughly 12uF per 100 watts of ballast but it varies depending on which type of tube/ballast is fitted.
I'll dig out my chart later if I can find it and confirm.
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc