Hi all
I am also new to airbrushing and forgive me for maybe asking a dumb question but is there a rule of thumb for thinning. At the moment I am dabbling with acrylic paint, so any advice would be greatly appreciated
Hi Big-aj and welcome to the forum, It all depends on what acrylic paint you are using. All paints thin differently, some spray straight from the bottle, if you can give us more information on the paint you're using then one of us can give you better guidance.

The paint I am using is Aqueous by Mr Hobby also known as Gunze Aqueous it is primarily modelling paint water based acrylic
I don't know this brand, but in general it's better to use paint specially designed for airbrushing they grind the pigment much finer for the small nozzles.
a good starting point is to reduce it to the consistency of milk, and then have you psi set at around 20-25........ from there you can reduce both the paint and the pressure depending on brush/paint brand/ need/ temperature etc etc

no real rule of thumb but milk is a good starting point
Thanks for all the advice, I sure as hell need it. So on that note I will now get down to practice my thinning, thanks again all.