Light distance and when to top?

Every light is a bit different. If you have an expensive PAR/ppfd meter, you can start at generally accepted safe levels relative to maturity and trial and error from there. with a cheap lux meter, you can guage intensity.. if X lux is too much for a seedling, you know to test lower than X lux in future. This will remain useful information as long as you use that meter under the same light -- or, the same exact light model. i.e. same spectrum, same diodes, etc etc.. $150 vs. $10. Your plants are probably ready for ~full intensity at this point or near that point. Start at 18-24".. let the plants grow into light a bit and look for adverse impacts. if it stops growing vertically or if the top buds flop over, you know it's too much light. If you see the leaves drooping for long periods of time during day, that's another minor sign -- they should rise and fall a bit, though. If i see my plants droopy for hours at a time near end of day i reduce intensity or raise lights a bit. how long you run your lights will matter too. check out a DLI chart. It'll help you determine if you are giving too much light based on specs and how long to run you rlights- - may be able to save some money at 16-18hrs instead of 24hours that some run them. https://www.ledtonic.com/blogs/guides/dli-daily-light-integral-chart-understand-your-plants-ppfd-photoperiod-requirements Use the chart, but don't follow suggestions for different plants.. mary jane can go upto 65DLI with ~1300ppm CO2 or so. Better to stick to 30-40 sans additional CO2. With that info you can look up the PAR/ppfd you provide at the hours you run the light and determine what your DLI provided is. So, it's going to take trial and error. You don't need a meter of any type, but it would help speed the process up. Waiting for 6+ nodes is safer. you can probably get away with it earlier, but i think you get less impact doing so. If the axilliary growth is starting to grow out just a bit, i think topping causes more "bush-effect" than if they are tiny little nubs in the nook of a node. you got the right idea... test somethign out... see how it goes.. adjust... take soem notes for future reference until it's second nature.