Yellowing Leaves and Brown Necrosis - Help?

Hey gang-

First grow and total noob here - be kind ;)

Fours weeks into flowering and what I thought would work itself out has probably become quite a mess.

I'm guessing a nasty Mg deficiency. Probably also Ca.

First off: AK clone, FFOF soil, CFL, small dose of bloom nutes at start of flower cycle. Water pH'd to around "medium green". No idea on PPM (it's city tap water if that could mean anything). Temps are 65 (dark) - 85 (light).

Just did a full (and first) plain water flush yesterday.

The leaves that are exhibiting this problem are from all over the plant so I'm guessing it's been with me for a long time. This ain't gonna go away before harvest, I'm sure. But, helping identify this problem now might help me with the four clones I have in veg for two more weeks.

Thanks for any help!

Sunny Girl Gardens