Epsom salt ppm?

I've seen a lot of people recommend 1 teaspoon per gal of epsom salt if u are having magnesium deficiencies or are using reverse osmosis water. When I disolve a teaspoon in a gal of water my ppm meter reads at around or over 400 ppm. isn't that too much magnesium aND sulfur for the plant. I have been adding around 1/2 teaspoon per 5 gal of water and getting around a 50 ppm boost. Not sure if it's helping or not. Main thing I'm wanting to know is will the 400 ppm plus my bloom certs burn plants or cause lockout?

most likely not . BUT many who use lucas formula with GH nutes do the same as you 1/2 to 1/4 t 50ppm is purrfect...
just be sure your ph and cal levels are there .. gh micro has no mg so it is often added...

I'm using botanicare cns 17 bloom which has calcium and magnesium in it I've adding 2ml calmag per gal since I'm using ro water

You should get around 0.2-0.4 (strain dependent) mS (mili Siemense, a measure of EC, electric conductivity) of magnesium and calcium in your water.
I wouldn't add excessive sulfur as it kills fungi in the medium. Maybe not at first but it occumulates. I would def. not reuse the medium for long.
I use tap water to get my magnesium or a cal-mag additive rather than a mag-S.