Just as an example –
If the results were 7000cfu/g and material is “clear” then assuming duplicate plates are spread for each dilution, density ~= 1g/ml, one would give –
Zeroth dilution (1g sample on plate) >> ca 7000 colonies on each plate >> not usable other than if desperate
One-tenth dilution (0.1g sample) >> ca 700 colonies on each plate >> not usable unless no other options
One-hundredth dilution (0.01g sample) >> ca 70 colonies on each plate >> usable, eg 60 / 80 > average 70
One-thousandth dilution (0.001g sample) >> ca 7 colonies on each plate >> not usable unless no other options
(Note - If the material is very dark, then if worst-case scenario, maybe only the one-thousandth dilution is usable > statistical problem).
So an ideal scenario requires 2 separate samples to give pairs of plates which both exactly average 70. It’s not impossible, just improbable.
If >1 dilution gives usable plates for the overall average calculation, the likelihood of identical results presumably further reduces.
(I am always cautious over such things because statistics is cunning, just like the chance of 2 people in a room of 30-40 having same birthday, one would think it's minute, in fact it's "amazingly" high)
Sorry, I have no personal experience on this. How do you normally decide when cleaning is completed ? pH ? (often used for CIP cleaning control from memory).
Micro-wise, it looks like a case of trial/error. eg getting an idea of APC vs flushing time. Presumably the initial level should rapidly tend to normal water level (~zero?) (or whatever lower limit the lab can reliably “measure”). The latter will depend on what their practical options are.