There is no one particular pH that is "right" for your input water, it all depends on the pH of your runoff, which reflects what your plant's roots are experiencing and is the result of the interaction of your water, nutes, soil mix, etc. So, potentially your input water could be at a pH that is higher than the recommended range for MJ but your runoff is too low, in which case you would need to raise your input water pH even higher to get the pH that counts in line. So yes, get a pH meter and measure your runoff, then you will know if you need to raise or lower the pH of your input water -- regardless of where the input water's pH is starting from.