changing oil on 1994 rm 125 forks

[shiftmx731]

hey guys..I have a 1994 rm125 and i changed the seals on them which was a snap, only I have no idea how to fill the oil to the right dimensions..they arnt like regular forks where you take the spring out and collapse the fork and fill to the top and suck out the 100mm....on my forks the spring and damper and all that good stuff stays in..the suzuki manual said to put 15.5 ounces in, but when i screwed everything on tight when i compressed the forks you got a swooshing sound...the manual says i need a little tool that holds the inner cap up while the upper fork tube is held a little lower and thats the correct setting for filling and checking oill, but I dont have one of these tools and so I have no idea what the ideal height of the upper fork tube should be to fill them..any suggestions would be appreciated. thanks
im also using maxima 5w fork fluid..if that means anything