amount of oil for shortened forks

i purchased a set of forks for my 900 they appear to be factory forks shortened about five inchs. i was wondering how much oil to put in them, they have some oil in them now but they bottom out easily. i would like to stiffen them up some. thanks
p.s. i'm not exactly sure what they are off of they look the same as my stock ones though.

got a picture of them?probably just have slugs in them,taking 5" out of stock forks would have header dragging on ground;)