Pro-Mix HP + GH Nutrient user here, checking in. Howdy!
According to the bag, you should start feeding within 7 days. I've also reached out to the manufacturer about calcium specifically as Pro-Mix HP is full of dolomite lime. They re-iterated to start the full nutrient regimen, calcium included, around 7-10 days.
I'm currently on day 19 of a grow -- I started feeding around day 9 or 10. In hindsight, I probably should have started closer to day 7...
Also, I start counting my days when the seedling pops up out of the soil -- this is day 1.
Finally, be careful with dosing GH nutrients - if you're feeding at 1/4 or 1/2 strength, you're probably underfeeding. Check out the GH website, you'll notice the feeding charts have been changed dramatically in the past couple of months. Before, they used to have two feeding options: (1) Recirculating; and (2) Drain-to-Waste. They now have 3 feeding charts: (1) Aggressive, (2) Medium, and (3) Light. You're going to follow the LIGHT feeding chart, closer to 3/4 strength during vegetative growth, and then closer to full strength come late bloom - again, the LIGHT feedchart (I believe this is closer to what was once labeled 'drain-to-waste'.
I tend to give plain water in between each nutrient feed, and water/feed every 2-3 days. Also best to keep the peat damp as to not kill off the mycorrhizae. Don't worry about overwatering too much, as long as you're not watering more than every 2 or 3 days. That's kind of the point of the 'HP' designation -- there's so much perlite in the mix and it's just so airy in general, your roots should be getting lots of oxygen. The first time I grew in Pro-Mix HP, I was waiting for it to nearly dry out, closer to how soil would behave. When your plant is young and not drinking much, you could be waiting a week before the pot looks dry! Don't let this happen -- just water again after 2-3 days, even if the peat still looks moist.
Also, get yourself good pH and ppm pens. Pro-Mix is closer to hydro than soil, so if you're feeding above 6.5 pH, you're going to run into issues. I like to keep my pH closer to 6. As for ppm, aim for 550ppm or so in your nutrient solutions.
-tronN00dles
