The Tetenal kit from my experience produces excellent results from the 8min/30c time.
Tetenal recommend the 8min/30c time in their instruction book if ~38c with the shorter time produces uneven results (assumedly from not enough agitation intervals in that short time period). So 30c is recommended for more consistent results.
I had my Tetenal producing great results for a year.
Do the spots wash off?