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I am not exactly sure that I understand your question, however I will offer this... 1) when using the incident metering system for exposing film you need to set your asa dial to 100 no matter what film speed you are using because in BTZS you are first after ev's for the Hi and low values to establish a SBR ...the chapter in the BTZS goes in to great detail on this issue. 2)The point of doing the personal speed point exercises is to establish a personal precise way of doing things that is repeatable so that when you want to test another film/dev combo or other technique you will have a very accurate point of reference. It really wouldn't matter which time either .25 0r .5 or even .4sec(as long as you can accurately repeat that time because you'll use it from that point on) if by setting your enlarger(light source)height, exposure aperture correctly you arrive at a personal speed point near 2.4. Deviating too far will begin to restrict your useful testing end points. 3) once you establish that set up and procedure you can very accurately compare other film/dev combos using your now established testing platform. If you choose to test and compare another 100 asa film you would use the exact same set up. When you plotted the data you could easily see if the new film/dev is faster or slower and by how much and also could compare curve shapes etc. If you want to test say 400 asa film you would use the exact same set up and procedure except this time you would stop down the aperture 2 stops or adjust the height to get 2 ev. You should not be adjusting the exposure time9once established in PSRP) because that would be the most inaccurate adjustment and the most limited due to reciprosity issues and timer/light source inaccuracies. Hope this helps if not sorry for all the words  .
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