Yea the flush will get rid of all the nutes, but ive also heard that foxfarms tends to get salty alot faster then most other soils. I know i personally like to use a light soil that doesnt have any ferts in it at all just because i dont like pre ferted soils messing with my feeding regimine.
If you wanna do a quick test water your plant till the point of run off, what color is the water? is it kinda a clear brown / yellow or is it just plain clear? If its tinted brown or yellow you need a flush, that tint is toxic salt build up.

As for your problem...im not really all that good at diagnosing, but your leaves look alot like my leaves did when i had PH lock out (the "burn" looks like ph spotting too, to me atleast). Do a Test on your run off water (assuming its not all dark and nasty like previously mentioned) you want it to be some where in the area of 6.5.

Here's a shot of what my leaves looked like during the ph lock out (they were also being underfed and overwatered too...god ive done so many horrible things to this plant).