where in exaust line to place helper fan?

I have an exhaust line that i'm thinkin about putting in a helper fan to help move air.

My question is where in the line should i put it? Right now my main fan is about 5 or 6 ft before the exhaust. Is it ok to put the helper fan in between lights? So if i have 2 1k's could i put the helper in between the lights?

Thanks the the input!

i would put one on the intake and one on the exhaust and have the lights in the middle, i think that would give best airflow