[quote]I have discovered a few web sites that will work out where the fret markings should be if you tell it how long the scale length is and how many frets you want to add. Will this actually work? or would be easier to measure the markings on an existing bass I have with a similar scale length.[/quote]
Similar scale length won't be enough - it would have to be identical.
I'd say use one of the fret calculator websites - or use a tuner and play chromatically up one string
[quote]Secondly, what would you use to mark the fret positions on the neck. I thought of very thin strips of tape but wondered if that would effect the sound?[/quote]
Thin strips of masking tape would probably be the best option, yes. Provided it's thin (in relief to the neck) it should not make a difference to the sound, as the action would have to be seriously low for them to get in the way!