You will always have some PSA, so undetectable is not equal to zero but really just the lower limit of the test.
Theoretically, the halflife is 3.15 days, so if you know your PSA at surgery, divide it by 2 over and over until you reach .01, then count the number of periods then multiply that by 3.15 days
Six weeks would be fine for you unless your PSA was a lot higher than you show.
I'd encourage you to get an ultra sensitive PSA, as you are really interested in knowing your nadir and when you reached your nadir. Hopefully, you will be measuring very low PSAs the rest of your life, so choosing the right measuring tool is important
<.01 is quite different than <.1
This may be interesting reading:
Fingers crossed for you!
Age 59 PSA: 8/09 - 5.7, 10/09 - 6.4, 12/09- 7.2, 2/10 - 7.4
12/09 FreePSA 10%
Biopsy 12/09 3/12 cores positive for 3+3 (10%, 20%, 20%)
Multi-focal Adenocarcinoma, 4th core Hi PIN
Path: still 3+3 clean margins, appears contained to 5% of prostate
Dry at 5 weeks, total ED corrected with TRIMIX
09/11 PSA <.01