I highly recommend you don't experiment with soil additives and i recommend even more strongly that you don't attempt to create your own. There are plenty of excellent brands that do a fantastic job. Best case scenario is that you will come close to matching the quality of a good pre mix, at worst you will fuck up your grow completely and get rubbish results. Seriously, unless you are an expert on soil makeup then you have nothing to gain by going well above your pay grade and attempting to create your own soil mix and everything to lose.
I don't know how closely you read my reply but let me re-iterate a very important point;
The number one advantage of using a good quality store brought soil mix is that you do not have to worry about pH. My tap water is even more alkaline than yours. My tap water is closer to a pH of 8. When i grow pot, i get a good quality soil and i feed it straight tap water for at least a month, sometimes more. Not a single drop of pH up/down (despite my high alkaline tap water) and not a single drop of plant food and what i get is beautiful thick lush green foliage from the bottom to the top. It's only mid way through flowering that i have to start adding extra fertilizer in my water (and even then, i do not need to adjust pH because the buffers in the soil take care of that).
I made the same mistake as you did by worrying about pH and adjusting my water so it was exactly right when it went in, and what happened was that by the time i was 5 weeks into flower my plants were suffering nutrient deficiencies because all of that pH up/down i was adding had fucked up the pH buffers in my soil. The result was an acidic soil which caused lockout and my yields and quality suffered for it big time.
For the situation you are in now, i would recommend a complete plant food with a decent level of nitrogen. Do not give it just magnesium and/or calcium - given the low quality of your soil it probably needs everything.
I get the feeling that you are going to do your own thing anyway, and that's cool. But i don't mind going to all this effort because people read these threads later, sometimes years later and this is the kind of advice i wish i had taken when i was starting out.