I thought, maybe I don't need quite that much calcium, If I add in a tiny bit of Epsom salts and drop my sulfate a bit, I could maintain my balance of chloride and sulfate, drop the calcium to around 40-50 and it would bring my Magnesium up to around 7. Since the acceptable levels of Mg are pretty low, I was afraid of overdoing it, not knowing what the actual Mg contribution would be from my malt. My feeling is, I'm probably OK adding the Epsom salts. What do you think?
There is no problem what so ever with a Mg level that low. A case in point is that almost all southern Bavarian waters have 10 to 20 ppm Mg. I'm doubting that anyone is going to say that beers like Spaten, Paulaner, or Weihenstephaner are terrible tasting beers. So the premise that ANY Mg in brewing water is to be avoided, is clearly false. However, it does rapidly become poor tasting in water when above 40 ppm, so there isn't a lot of latitude in dosing.
The problem I find is that a brewer adds epsom salt to a tap water that already some Mg in it and quickly over runs that upper limit. Be sure you know where your Mg level is before ever considering adding it.
While I am a fan of tasting ingredients by themselves, be aware that in the case of water minerals, adding a pinch to your mouth probably puts the concentration of those ions in the parts per hundred range. That is a bit higher than the parts per million concentrations that may be desirable in brewing water. Trying to draw a conclusion about epsom salt's contribution from tasting a pinch is not going to be telling. Remember, the difference between a medicine and a poison, is dose.
Minor concentration of Mg in brewing water is OK and it definitely is desirable in hop forward and bittered styles.