I'm certainly no expert on the subject but I can share what works for me.
I primarily use ModelMaster Enamels and I haven't found one "golden" thinning ratio. I think the viscocity varies slightly depending on color and how old/well mixed your paint is. With my rather basic airbrush Aztek 4709 I use the finest beige tip and low psi (around 8) on my compressor. I mix a thinner/color mixture that is the standard "milky" consistency and then I add thinner drop by drop until I find a mixture that creates a faint line without having the paint being blown away by the airpressure (I use styrene strips as test surface). 1.5mm doesn't sound all that bad to me but it depends on what effect you are going for. I try to slowly build up the intensity to create a shading rather then a pronounced line. When I do need something more pronounced and very fine I usually just tighten the line by carefully putting down lines of base coat on either side of you desired area thus making it finer. This will be harder on a more complex scheme like the Luftwaffe camo but could be useful on the wings and undercarriage. I'm sure the more $$ airbrushes out there can produce finer lines but this set up has worked well for me.
I glued the fuse and wing of the 109 last night and got a surprisingly good fit so now I'm quite excited about this kit.