Here's how i do mine. Hope this will help in some way.
MANUAL FLASH MODE : In my AF 360 FGZ, i tried to measure the flash and it gives me a reading at Flash power at 1/8, distance 2 meters, (K-20D,K-7,K-5) at F8, ISO 200 using FA 31mm ltd. And then using Sigma zoom F2.8, 24-70mm at F6.3 (with the same setting) . Remembering the difference between F1.8 and F2.8 lenses at least gives you an idea how to adjust either the flash power, bigger aperture, or pushing the ISO up a bit and its your choice which one will suit you best.
The reason I use manual is that it produce consistent output disregarding the luminance of the subject outfit.(this may not 100% foolproof, but at least you have the basis in simple math.
( with few adjustment flash compensation). I shoot with umbrellas and soft boxes and always get the exposure needed.
Also I use two flash gun but most of the time ended up just bouncing the master on the ceiling if applicable.Using a Master(Sigma EF500 FGZ bounce up) and Pentax AF360 as slave in P-TTL in 1/3 or 2/3 power also provides good contrast.
Im not really sure how the pentax manage to produce a decent flash output using ISO all the way up to its unexpanded limit ISO 12800 (.... haven't tried the expanded yet)
The built in Flash actually gives me an acceptable exposure even using ISO 2500 in few pictures
Flickr: Please wait...
( My K-7 provides me at a distance of 1.5ft at F14 and ISO 200 built in flash -0.7 1/180 consistent output )