Hi Adrian, I spray quite a few of mottled camouflage schemes. They are amongst the most varied 'paint jobs' you can attempt. I certainly take my hat off to you for having a swing at one first up. I think you've done very well.
The trick really is that there isn't one! Once you get your
airbrush working well with your combination of
airbrush, paint and pressure use it to spray everything. There is no easy or trite reply that I or anyone else can give you to achieve this. My one advice would be to up your pressure. A lot of modellers seem to spray at pressures lower than those at which many airbrushes work properly. Have a go at 35 psi and don't be scared to up it a bit and see how it goes. After that you'll just have to fiddle with your thinning ratios and then practice, practice and practice a bit more.
Always remember that these varied Luftwaffe schemes, mottles, botches, squiggles etc were applied by humans using spray guns. To keep in scale try to envisage how far they could reach. Also, many of them, particularly late in the war, might not be very skilled sprayers and hence achieve less than perfect results.
This one is early and was a fairly neat job:
Cheers
Steve