It looks like you're describing a piecewise function. I doubt there is "one formula to rule them all". Assuming that the "slidy" part is linear, you could maybe try something like this:
y = 40 for x < 1500
y = 40 + ( 3 / 2350 )( 1500 - x ) for 1500 ≤ x ≤ 25000
y = 10 for x > 25000
Rigil