Sounds like a great candidate for a small partition for veg and a 1000HPS on a light mover! lol
I hope you don't mind me chiming in, I had my share of PH issues when I first started gardening. Gypsum is a great addition to a garden because it doesn't contain nutes, but it does help the root zone to speed up the processing of organic materials for absorption. Basically, gypsum makes it so nutrients are made more readily available to the plants as they demand them. I think the concept they are trying to illustrate consists of two parts. First, the gypsum in soil also improves drainage, which makes a flush more effective. Second, the application of gypsum frees up unused organic sources of nitrogen, also making the soil more acidic. If I had alkali soil at 7.8, I would do ONE of these three things.
1. Let the soil dry completely, PH watering solution to 6.3-6.6 and top dress with compost or composted manure. Give a heavy watering with plenty of runoff after the dressing. By plenty I mean around 30% of the water you put in comes out the bottom of the pot. Filter runoff with a coffee filter and check the PH.
2. Apply sulfur, carefully. If it takes two or three waterings to reach your target, so be it. This is risky and you don't want to overdo it and go from too alkali to too acidic overnight. This is a touchy method, but also the most immediate method to make the soil more acidic. After sulfur is applied, filter runoff with a coffee filter and test the PH.
3. Flush - last resort - Take the volume of the container and multiply by three. Run this volume of PH adjusted watering solution through the plants. You could use gypsum here but i've never used it for a flush. Once flushed, feed at 50% strength appropriate to plant cycle and wait. The next time you water, filter your runoff with a coffee filter and test the PH.
I wouldn't be too concerned with 7.8 and I would opt for option one. The other methods would be appropriate if you were further out of range than you are currently. And if you're considering using citrus juice to lower your PH, it works great. I have a friend that opts for citrus juice over PH down and has had great success for a cpl of years. The trick is calculating how much you need for your amount of solution. Document the effectiveness to dial in your application levels. If you go too far, add a little baking soda. You could water with PH solution of about 6.3-6.6 until you get back into your desired range using method one.
Another note, MG soil is not recommended. Have you ever tested the PH of the MG soil? I'm curious what it is out of the bag. You did the right thing by cutting it with perlite! I would also cut it with some peat, coco or Foxfarm Light Warrior. This will help improve water retention, drainage, reduce the hotness of the MG soil and also lower the soil PH a tad. MG tends to get pretty hard, I learned the hard way that although its cheap and highly available from local vendors, that its just not worth the savings or the convenience without modifications.
Glad to hear your PH kit is on the way. Its probably the most essential tool for indoor gardening.
Lovely looking buds! What size of container is that in? I really like the spectrum of CMH bulbs. I ran a 400 CMH with a 600 HPS in a 4 x 4 and the results were good. The CMH doesn't stack up in lumens compared to HPS, but the more ideal spectrum seems to compensate for the compromise. The heat output is fractional compared to HPS, which I really liked in the summer. I didn't care for the beastly 400 magnetic ballast though. Do they have CMH that runs on digital yet?
