I have always treated manufacturers film speed and developing times as a reliable starting point - rather than the final destination.
If you have a reliable and consistent development method there is absolutely no reason why you cannot make your own personalised adjustments to both film speed and development times.
I have conducted some Beyond the Zone System film speed and development tests for both HP5 and Delta 100 in for both intermittent agitation (rolls of film in a Paterson Tank) and continuous agitation (sheet film in a Jobo CPP).
The results look nothing like the manufacturers recommendations.
Martin
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