Yes I would lower the pH now. From what we have more recently learned, the pH test is actually not inaccurate at first even at higher FC levels when there is CYA in the water. At high FC what you will find with the pH test is that it initially gives the correct reading and then over about a minute or so it turns the purple color caused by the high FC oxidizing the indicator dye. Apparently, the lower active chlorine level from the presence of CYA slows down the rate of the FC interference in the pH test. So your pH reading of 8 is likely correct and you should lower the pH quite a bit (at least to 7.2 if not 7.0) before shocking.

As for the yellow/mustard algae shock level, you don't have to be exact about this. 35-40 ppm FC is fine. It isn't like there's a magic number where it works -- higher just goes faster.