I've used to shooting powder for years always had great success I still do , side by side blows any other finisher out of the water because it's available instantly. Most finishers Take 2 to 4 days to break down and allow the phosphorus and potassium to be uptaken, where the shooting powder is...
Looks like a magnesium deficiency.House and Garden has 11% calcium is the base already, by adding extra calcium you can block out calcium and magnesium. I would say just add magnesium. I use House and Garden and I found with some genetics they need extra magnesium