the function is

f = 32400/x


because:

a / 270 = 120
a = 270*120
a = 32400

test:

32400 / 320 = 90
32400 / 90 = 320
32400 / 120 = 270

Some small recent games of mine:
Griefer 4k http://damocles11.byethost9.com/griefer4k.html
Kobold 4k http://damocles11.byethost9.com/kobold.html
Cave 4k http://damocles11.byethost9.com/cave4k.html