You are correct!
I forgot to factor in the required yield - from autos (experienced grower, good breeder/genetics, and heavy yielding strain), you'd need two medium plants to produce the 140+g to keep you going while your next crop is growing, but if you have enough juice light wise, you could probably push 200g out of a topped, lst/scrog'd regular/photo period plant in a 70x70x160, but then again, if grown from seed you add around a month onto your grow cycle (auto vs photo), so he might run out before next crop is done, you also have to factor in 7 to 10 days of drying and 45+ days of curing (minimum I cure before I start smoking my buds).
That's the one thing about growing cannabis, one grower can achieve 200g from the same clones as the guy next door, who only achieves 60g, so many factors that come into play, so it makes it a bit hard to guestimate yields, but working off of my experience, a very cheap setup and beginner grower can get at least 50g of decent quality bud from a grow, barring any major incidents along the way (pests, deficiencies, toxicity, etc).
Then lastly, a bit of research always makes a huge difference... in my opinion it's very beneficial to understand the stages of the plant, effects of light intensity and spectrum, understanding how plants update nutrients and how growing in different grow mediums have an effect on the buds in certain aspects (sweetness & taste, etc), such an amazing herb.
Anyhow, all the best and happy growing!