There is "a science" to the decline in PSA that I'll help to explain. Doctors probably don't do these calculations for each and every patient, but they probably do the rough math in their heads each time. They probably do something like this: "Wait 4-weeks for someone with low PSA (like you), or wait 6-weeks or 8-weeks for someone with higher PSA." Other docs might just wait 6-weeks for everyone. Here's how it works (please forgive me, but to save time I'm going to just copy/paste from an earlier post of mine):
There is a "science" behind the level of PSA decline after surgery...I'll explain how it works, but to answer your question there is no specific "normal psa 6 weeks out." It depends primarily on one's PSA prior to surgery...the starting point. Once the primary source of new PSA is removed (the prostate and seminal vesicles), then the PSA already in one's bloodstream diminshes over time.
Studies have been performed which show a distribution of how fast PSA diminishes, but in most men the "half-life" of PSA in one's blood after RP is 2-3 days. So, if one's "starting point" at the time of surgery is 5 ng/mL (close to yours), and if one assumes a 3-day half-life, and if one assumes no additional source of PSA added into the bloodstream, then the PSA level would reduce like this:
5.0 ng/mL at time of surgery
2.5 ng/mL 3 days later
1.25 ng/mL 3 days later, 6 days after surgery
0.625 ng/mL 3 days later, 9 days after surgery
0.3125 ng/mL 3 days later, 12 days after surgery
0.15 ng/mL 3 days later, 15 days after surgery
0.08 ng/mL 3 days later, 18 days after surgery
0.04 ng/mL 3 days later, 21 days after surgery
This is the "science" behind why surgeons prescribe the first PSA typically 6-8 weeks after surgery (some very conservative doctors want to wait longer...the longest I've heard of is 3 months). There would be some statistical "outliers" to the 2-3 day typical half-life which would result in a longer time to diminish.
Of course, this half-life decline described above assumes no PSA is being added back into the bloodstream. There are, however, secondary (minor) sources of PSA, however, which prevent any living, breathing man from every achieving "zero" PSA...the phrase "zero club" is a misnomer which sometimes causes undue worry in men when they hear that their PSA is something other than "zero." Prostate cancer produces PSA. But a common source of PSA after RP surgery is the neurovascular bundles which may have been safely left behind during surgery; they do produce a low level of PSA, and the amount produced varies from man-to-man.
BTW, I thought Ohio State's comment was actually good advice if you still want to add more "margin" to your result...