Ph water.

pH is always the LAST thing you do before ALL water/feeding. It's one of the most important things to do.

You not only pH every watering, you need to pH every nute solution you make up as well. M0use's ranges are spot on and she's right - if you're growing hydroponically, it's the same pH range as soiless. Good luck!

There is no best, there is optimal range. Each strain will have slightly different range that it has become accustomed to and will have its own sweetspot based on what country it was native to. First you should know Ph is logarithmic, much like earthquakes so a magnitude 6.0 quake is 10x that of a magnitude 5.0. 7.0 is 10x that of a 6.0. Same applies to pH. Distilled water at room temp is 7.0 ph(neutral) Distilled water at boiling point 6.2 ph Doesn't seem like alot but boiling water is 8x more acidic than room temp. We all know NPK the big 3 but every single element has a different optimal range at which the rootzone can uptake. I forget exactly but what uptakes well at 6.0 (calcium manganese etc) is almost non existent at 7.0 So many times I see people staying religiously exact. When from my experience and research it seems what you should do is oversaturated your plant with light. Once photosynthesis has stored all in it can for the day cycle the excess light is used by the plant to pH the medium itself based on what nutrients it needs at trying stages. It's called leeching. 90% issues come from using artificial fertilizer derived from crude oil by-product of petroleum.

Depends on your setup, but yes every watering you PH adjust. Soil grows 6.4-6.8 is my target Soilless is 5.8-6 in veg and 6.1-6.3 in flower. Hydro I don't grow but I think its the same as soilless. If the nutrients are available immediately PH anywhere from for 5.8-7 should work in theory but other things other then nutrients are affected with PH and Temp of the medium. Good Luck!