Why can’t you use epsom salts when using dolomite lime? I’ve seen this many time but never an explanation.
As bill nye would say ……when a low soil pH is desired, or when making the gritty mix, there is benefit in using CaSO4 as a Ca source in lieu of (Ca/Mg)CO3 because the CaSO4 doesn’t raise pH. Whenever you do chose to use CaSO4 instead of Ca/MgCO3 as your Ca source, you need a Mg source and MgSO4
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I’m assuming its ok to use calmag with dolomite lime in my soil?
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Maybe. It’s going to depend on what kind of nutes, water, and soil you are using. Dolomite lime is a good source of calcium and magnesium but it’s kind of slow acting. It also raises your pH so you don’t want to get too crazy with it. You also want to mix it in thoroughly Top dressing or watering it in ain’t gonna cut it. Cal-mag has iron as well. Dolomite lime doesn’t.
Dolomite lime is fairly slow acting. Too much of it in your medium will lockout other nutrients so it’s not a good idea to add cal-mag in this way. It needs to be mixed in the soil prior to planting in order to be effective. Even then it may be too slow acting for cannabis and you may still have to add some sort of cal-mag supplement.
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I only mixed in 2 cups for 3 cubic ft of soil
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Good info brother 
Generally you want to use 1 cup of dolomite lime per cubic foot
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I agree but im using ocean forest which already has oyster shell. The reason I add lime is because I was having low ph issues starting about 4 week into flower. With already having the oyster shell I think 1 cup per 1.5 cubic ft should be plenty.
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I use ro water so I use calmag every water which probably isn’t necessary with the added perlite and lime but I use it to add minerals back into the ro water to stabilize the ph. My RO water ph is 6.4 and tds 10 but after adding 2ml of cal mag its 7.0 ph and 120 tds. Which leaves me adding 2 to 3 drops of ph down. That gets my base water 6.2 ph 175 tds.
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The simple answer is dolomite lime already has magnesium in it, adding more will throw off the calcium to magnesium ratio. If you find you need more magnesium, use a product that also has calcium in it.
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I understand…im also using calmag @ 2ml per gal because of my RO water. If I dont add the calmag ill get huge ph swings because of the lack of minerals.
Would I be better off using my tap water @ 16 gpg hard (calcium) ph 7.5 and tds 250ish?
I guess either way I’m adding calcium though. Pretty hard to over do the calcium i suppose.
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I use tap water (very hard), it comes out of the tap at a pH of around 8.1 and a PPM around 250.
But I stick with RO for my living soil beds.
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The last thing I want to do is use my RO water with salt based nutes without adding calmag to 100 tds. Not enough minerals and ph goes all over the place.
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I know it may sound like a wishy-washy solution, but adding RO to your tap water would give you a better approach, less TDS, less pH and more minerals … 
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I thought about that as well lol. Grow delema I guess 🤷