Start with your 25ppm tap water. Gradually add calmag until the ppm of the solution reads 300. Then proceed as normal- use this water if you are doing a water-only feeding or adding nutes to it.
Set the bags on a grate like a cookie drying rack and slit some more holes in them if they drain crappily.
Lime in the coco worked last time because it added enough minerals to make up for your soft water.
Don't consider CalMAg an 'amendment' or 'supplement'. It is easier to think of it as 'concentrated awesome plant water'! It gives you the ability to make a water that is exactly right to form the base for any other nutes you use. Like if you want to make a cup of hot cocoa with just powdered cocoa, but you only have water, well first you have to add powdered milk to the water or it's not going to taste right. The CalMAg is like the fertilizer equivalent of the powdered milk. Once you have reconstituted milk, you can make hot cocoa or cereal or just drink it. Once you have reconstituted plant water, you can add grow fert, or bloom fert, or just give it to your plants. Get it? Coo.