I usually use the same film all the time to avoid the problem I have now. I was out on a field trip and someone traded me a roll of 125 iso film for my roll of 400 iso Ilford HP5+ just for grins. I changed my ISO on my Pentax MF camera and shot the 125 iso then later put in a roll of Ilford Hp5+ to do a portrait of my brand new granddaughter. I realized what I had done after the roll was completed. So here is the question. I'm thinking I need to pull the processing one an a half stops but don't know how to figure the time. Normally I process in Ilfotec Hc 1-31 at 20 C for 6.5 minutes. How do I figure out how many minutes to develop the film shot at the wrong ISO? Sorry if this is too basic a question but I need to know and where else would I go for the answer but my learned friends on APUG?