Playing the audit "lottery"  (then it always occurs to me that my clients are too small to bother the IRS) is unwise, and against the various rules governing your CPA license. 

There's voluminous material discussing whether a percentage of gross, or net, or distributions is allowed.  And the material pretty much all says 'nope'.

YMMV - but that doesn't make it correct.  Nor does the fact your new client hasn't been audited.  Yet.