I don't have any of the recent Citadel paints - mine are from the era when HMG made them in the round 18ml or hex pots...

I think it with ~50% vallejo airbrush thinner to keep the strength in the bond and not weaken the paint too much. I mixed a whole pot of Citadel 50/50 and it works a treat.

I would also never thin water based paint with alcohol because it will flash off even faster than normal and not really retain the resin consistency in the body of the paint - flakes, poor adhesion etc. are likely, although not experience by me as I don't do it.

What I also find is that they can be suceptible to tip dry on the needle and as long as I know this and keep it clean then not an issue, although when I first painted a Landraider I was getting inconsistent finish until I realised this was happening.