No 80% PP available on a Mk1 (no 'a') Spit there Todd! Only coarse or fine pitch available.
Guys for a good explanation on Spitfire props and the general differences between fixed, variable and constant speed units for all aircraft watch this video;
NOTE: This video details the A2A Simulations Spitfire for FSX and cannot be translated directly to CloD, in particular the current two pitch prop modelling for the Spitfire MkI in CloD is incorrect as you cannot tweak between coarse/fine pitch; takeoff in fine and anywhere above 120mph go to coarse. Fly level to pick up speed, get to 160-180mph indicated then start your climb.
Yes, in coarse at the lower end of the airspeed range you will have crap revs and power - thats just the way it is! So keep your speed up.
As rule of thumb - with a CSU you set a desired RPM for the engine and the prop endeavours to maintain that RPM whatever speed/throttle setting... IF IT CAN.
With a two pitch/variable you must adjust the RPM depending on your speed/throttle - Speed goes above 120mph, prop lever comes back is a simple rule. Your aim is to prevent the rpm exceeding 3000rpm.