Ideally you need a gauge (in line, or one that bolts to the FK) to set it up properly - don't leave the gauge installed though. You can get one for a few quid of eBay, IIRC.
Webers need about 4-5psi constant, I think - so you'd need to aim for that - or you can do it by starting from wherever you are now and constantly tweaking it until you start getting flatspots or poor acceleration, although not the best method.
It shouldn't really leak though, although they are prone to it and bloody irritating things for it (why I ditched mine and went back to a mech system. The filter king just turned it into a weaker engine all over). Might have just been a bad one though.....
Get some PTFE tape (make sure it's a petrol-proof one) from a plumber's shop (not sure if Halfords do it) and wrap it around the threads on the bung and reinstall it. If it still gives grief, I guess you could always try some petrol-proof sealant from Halfords or similar.
Hope that's a bit of help