Plant Veg Height?

I always read 18-24" plant height for flowering well not always being some 12/12 from seed, but on the height would you measure the pot up to the upper most tip, or medium to the upper most tip? I know sounds super noobish, but I'm sure this would help someone else as well.

Measure from the soil. It's the only accurate way.

Yeah that's what my line of thought was, was talking to a few people, and they argued that the roots are part of the plant, and if a plant is about to be replanted, the pot/container would be full of roots. Then I asked about hydro, and they had no answer. lol Just figured might be good for someone new to find.