Plant height to root ratio

i am trying to grow a golden privet tree to about 2metre high

(to match a mature specimen in my border which is shaped into an umbrella shape)

It currently stands at 1.2 metre in a 35litre container . I use containers and rootbags as border is close to neighbours house)

I was wondering if there is some kind of formula to pot size and plant height - (you know, something like height required x 2.5 gives litreage of container)

any ideas?

thanks