Hey
@Cobain420!
I just completed my first grow using Pro-Mix HP and the GH Flora Series of nutrients. You can check out my blog post
here.
I previously used Fox Farm Happy Frog, and I must say, I prefer the Pro-Mix HP and will probably be my go-to going forward -- until I make the leap to hydroponics of course

. Anyhow, I'm currently on day 21 of another grow with Pro-Mix HP and GH Flora.
Now, to answer your questions, follow the feedcharts here:
https://generalhydroponics.com/resources/floraseries-feedcharts/
*IMPORTANT*: Those feedcharts have been updated recently and probably don't match what's on your bottle. Previously, GH had a "recirculating" feedchart, and a "drain-to-waste" feedchart. They recommended following the "drain-to-waste" guidelines for soil/soilless, peat-moss included. Please note, if the feedcharts on your bottles are unlabeled, they're most likely the 'recirculating' chart. Regardless, both the 'drain-to-waste' and 'recirculating' programs are now OBSOLETE. The company has updated their feedcharts and they now call them 'aggressive', 'medium', and 'light'.
To recap:
OLD/OBSOLETE FEEDCHARTS:
- Recirculating
- Drain-To-Waste
UPDATED FEEDCHARTS:
- Aggressive
- Medium
- Light
For autoflowers, you're going to want to follow the LIGHT feedchart, scaled to 50-100%. Forget this 1/4 strength stuff and go by ppm. Aim for ~550ppm in late flowering. For me, I am currently following the LIGHT feedchart with early feeds at 50%, then 75% toward late veg and finally up to 100% in flowering.
Also important: start feeding around day 7 and certainly by day 10 at the latest. This comes straight from Pro-Mix tech support.
Now, here's where things get a little tricky: it seems like GH may no longer recommend the Trio+cal mag. They now provide 3-part, 6-part and 10-part nutrient formulas. I'm thinking this has something to do with what
@Mañ'O'Green linked to above, in that very worthwhile read on "balancing" nutrients. GH's Cal-Mag is also a source of nitrogen, albeit not too much. Anyhow, just adding Cal-Mag to the trio means changing the nitrogen ratio in the solution. As such, the next complete/balanced formula that includes Cal-Mag is the 6-part solution. Long story short, you might want to look into getting the other two nutrients in the 6-part line up: Floralicious Plus and Liquid Koolbloom -- that way you can rest assured your N-P-K ratios are properly balanced. Personally, I use the full 10-part line up, and the results speak for themselves. Having said that, the 10-part mix is not cheap and frankly, when it runs out, I'll probably look for something a little better suited for soil/peat, without so many bottles to mix.
Finally, a note on feeding frequency and pH
I personally aim for a pH of 6. That usually means somewhere in between 5.8-6.3. Don't forget the mix itself is buffered with dolomite lime and your run-off will pretty consistently be close to 6.9, regardless of what's going in to it -- this is why I don't bother measuring the run-off, rather just the nutrient mix going in. This is also why I'm a little more comfortable going as low as 5.8, where as with raw peat you'd probably want to be closer to 6.3. Anyhow, you will definitely start seeing pH problems if you're much higher than that. The 6.5-7 range is pretty well a no-no.
Finally, GH recommends giving plain (pH'ed) water at least once every 3 feedings. Personally, I give plain water in between every nutrient application, for the most part. Sometimes I might give two plain waterings in between feedings, especially early on; I might also give nutrient applications back-to-back. It really depends on how the plant is behaving.
Happy growing,
tronN00dles