Dear kbyrne,
As tsmith noted, there are typically several factors involved, eg actual equipment characteristics, target bacterium for calculation of xD, etc. The latter may also depend on yr location, (eg Salmonella vs. L.mono. vs ?)
I (tentatively) deduce from yr last post that you still have a procedure / data from previous "validation" but no source reference to "back it up".??
If so, perhaps you could post the (basic) method you used which was presumably optimised for yr system. This will help any searching through this forum (I can recall a thread here linking to a specialised US "roasting nuts organisation" with their procedures [some free, some not]).
Rgds / Charles.C