My first run

Hey guys I'm just starting off my first run and I've got a couple questions for you guys. I've got 2X 1000W HPS hoods with dimmable ballasts over 2 2'X4' trays which will have 4 Gelato's under each one during flower. I'm running in Coco and feeding 5g/gal VegBloom for nutrients and supplementing with 2ml/gal CalMag. PPM's come out around 850 after VB and CalMag is added and I pH to between 5.9 and 6.1 and I water each plant 250ml achieving good run off every time. My VPD is off slightly from where I would ideally like it but Im trying to keep it as close as I can with a small humidifier in the room.

My first question for you guys is concerning the yellow tips I've got on a couple of my clones in the room, its really only noticeable on the on I posted the picture of but when I really looked I could see it starting a tiny bit on one or two others, am I over/ under watering?

My second question is topping, how long should I wait before I start topping them? They were just transplanted into solo cups from the cloner a week ago so I think I'm still early but I don't wanna wait till it's too late, I have somewhat short ceilings in my room (About 5 feet from the top of the plants to the lights currently) so I want to keep them on the shorter side.

Feel free to drop any suggestions/questions too, this is my first time running my own little room so I'm open for trying things others have had success with!

Answer to your first question is that its likely nute burn 850ppm is hot for baby plants. Second question doesnt have a straightforward answer some only wait 3 nodes or so and some wait till 6-8. Look up LST, fimming and supercropping i like to use a combo of all techniques at times.