WeGroanswered grow question 3 years ago top and fim puts out a great deal of stress so what i have read:
it is advised to top/fim/train when the plant has 5-6-7 internodes, becouse at that time plant also has a good developed root system so the plant is not stunted to much..
from what i can see it looks like your plant is at 5-6 or even 7 internodes and now is a good time to top/fim.
(but you should probable try to fix your nutrient problem first? as that 1 plant is already stressed as it is. But its your call.
before you start to dabble with nutrients, make sure to measure the pH of the run-off water, an incorrect ph is most likely causing your deficiency, since the plant is young and you used fresh soil() that should still have nutrients in it.)