Drug/Grocery store Epsom salt is all you need for MgSO4, just be sure to buy fragrant free. Whichever you choose, RO or Distilled water, stick to it. If you are happy with distilled now, don't change up your water later on. It is best to get to "know" your water and keep it consistent. Your bicarbonate levels should be fine as long as it doesn't throw a warning. Which you may be trying to get the pH to low in your brown profile. Roasted/Darker beers are tastier, imo, in the 5.4-5.6. However my PITCHBLACK beers get 5.6-5.7 So if you are using acids to drop the pH to 5.2 that may be your issue. If not, then I say screenshot/copy&paste your Adjusted Summary on here and smarter guys than me will help you out.
Martin will eventually be on here to help you out if you get really stuck. He is always helpful. Just be sure to really read through the introduction and getting familiar with Bru'n Water you may find the answer to your problems there. It is an AMAZING tool - Thanks btw Martin.