Yup. Another one of these threads. You know the rules. I am pretty sure it's Phosphorus, would just like a second opinion. The fact that a couple are on the inside of the leaf and the fact that I use Cal-Mag leads me to believe it isn't Mag def. That usually forms all around the edges. It also is formed on a few leaves at the base of the plant so I am not super worried. I am growing hydro so I doubt it's splash burn.
Yeah, between 5.5 and 5.8. Sometimes drifting up to 6.0 but never higher than that. I check it daily.
If it's just a few leaves on the botom I would'nt worry alot about it unless it gets worse or the entire plant looks unhealthy.