Generally, drawdown isn’t a hard-and-fast number, though some guidance documents or permits do require a maximum drawdown of 3/10 of a foot or 1/10 of a meter.  But studies have shown that the key is to attain a stabilized pumping water level during purging, not at a specific drawdown limit.  You may have as little as 1/10 of a foot of drawdown, you may have as much as two or three feet of drawdown, but once that water level is stabilized, studies have shown that the indicator parameters will follow right behind.

As for good parameters, experience and studies have shown that dissolved oxygen and electrical connectivity, or specific conductance, are the most reliable parameters for telling us when a well has been properly purged and is stabilized.