The main issue for ball drilling that is rarely followed out, is to use one specific ball driller for a minimum of 3months.
During that time you would want to drill at least half a dozen drillings until u both come up with whats best for you
Rarely a Ball Driller will measure someone's hand and get it perfect first up
Look at all the variables with spans, pitches (both fingers and thumb) whether a wrist guard is used or not
I believe in what Clint has said is pretty close, there are several competant Ball Drillers in Perth now....
Simply choose one, but give who ever it may be a chance, too many people hop from ball to ball different drillers, then wonder why there bowling is inconsistent, generally blaming each driller along the way
Be like changing ur coach from month to month that wouldnt make for consistency at all
Hope this helps