Phosphorylation Assay Conditions

[bmbrl at biovax.leeds.ac.uk]

I'm in the process of setting up an assay system to see if a membrane protein
I've purified is a substrate for cAMP-dependent protein kinase (PKA).
Firstly, what concentrations of ATP should I be using?  If I add [32P]ATP to
the reaction mixture the final concentration will be ~0.1uM.  Would this be
enough to indicate preferred substrates for phosphorylation?  
Any information would be gratefully received.
Thanks, Rob.