This evening my sister gave me a roll of 120 film labelled HP5+ so I could process it for her.
I souped it in HC-110 dilution H for 10.5 minutes, agitating once a minute (standard practice for me).

It turned out to be Delta 400...and the negatives turned out absolutely fine. Not a problem.
I looked for dilution B on Massive Dev Chart, which says 7.5 minutes, and doubling that for dil. H would give 15 minutes...

So, to what extent is accuracy actually required when processing film? Was it to do with the dilution of my developer?