Hi,
This is my first grom with Pro-Mix HP plus 25% perlite.
Plants were on cup for about 15 days ten I transplanted to 5gal pot. On my first watering at pot I didn’t took runoff ppm reading but on the second my solutiuon going in was 350ppm and runoff was 1000ppm.
Why that? I never gave them more than 350ppm. Always with 10-20% runoff.
When on cup solution was 350ppm and runoff about 500ppm.
Should I flush it?
Thanks in advance for your help.
Are u phing?
Yes. PH in last feed was 6.1 runoff 5.4
Pro mix has some charge in it so what ever the soil has plus what your feeding can be quite high at first run off or 2
I am no expert but I am growing in Promix bx which is pretty much the same.
Your PH is gonna be MOST important. Promix likes more of a hydro ph than soil. I keep mine at 5.9 ish I’ll float up and down a bit.
Right now all my plants are happy and healthy except for my damn spider mite war. Which I am winning.
IMO if you’re just now up potting to 5 gal pots then your girls are too small for runoff watering. You’re gonna drown them. Ask me how I know 
Anyhow. Others will step in and help I’m just giving my 2 cents.
Good luck 
Just picking up from media in pot.