Anywhere between .1 ppm(as logn as you adding it often, this is pretty lean by most standards..... and 10ppm seems fine, Davis tap water had 52ppm with a range of 30-78ppm out my tap. Very high.
Tropica master grow trace has a little added.
PMDD, long pre dating PPS pro and curiously similar in virtually all respects, adds Mg, EI adds it, ADA adds it , every fert routine does...........
Mg deficient plants express is in, Anubias tend to get this weird yellow veining, not as dark green. Some others can get twisted leaves or progressively smaller stunted growth. Can look somewhat like CO2 deficiency.
General experience dating back 15 years even at high lighting suggest 1/4 teaspoon of Epsom salt per 20 gallons 2x a week is plenty. You should check your tap water. The potential for Ca deficiency is generally very low with Tap GH's, but more common with Mg.
Generally, adding a multi range GH booster is preferred these days, but you can add just MgSO4 if you want and know the Ca and K+ are fine. This adds Ca, Mg, K+ and is done just like a water change routine with tap that has a good GH, except you add more GH than what's there in the tap to ensure no deficiencies.
High GH has(say up to 9-10) no adverse impact on any plant I am aware of(and having grown 300+ species to a high level of healthy nice growth, I'd okay with saying that). Many of my tanks and plant species I grew had even higher GH than that, but I only grew perhaps 70 species in GH tap water that was 24 degrees.
High Ca+ is not likely to cause issues, that I do know, but really high Mg might(or might not).
Most folks have high KH if they have high GH also, that, rather than KH is the issue.