Hi there pharmaboy!
I think that the timing of your shot will be a bit machine specific. A twenty five second shot from my little Sunbeam with a tiny portafilla is going to be vastly different to the same length of time from the commerical machine I work with.
So, I think I have seen it said here somewhere before, go with what your tastebuds tell you!! ;)
I havent changed a shower screen either, just have given it a good clean and backflush as required.