v2001,

is y=10% when x=40% all that is needed? that might be sufficient to fit the curve (mathematically) but doesn't sound technically right to me. there must be other requirements. if not, it's a strange problem.

it took me a while to figure that when you said, x is 40%, that's a value not percentage change.
an equal percentage curve is such that for the increase of x by same amount (in number, not in %), y changes by equal percentage. that is nothing but an exponential curve.

y=exp(x)
 
for your case, the generic curve y=Kexp(nx) +C should work. it's just the matter of finding the right values for constants for your problem IMO.


khaobadi,
it's not 10% or 15% increase, but 10% or 15% open (valve that is, in this case). And your second example does not yield same percentage change in y (flow). That is not equal percentage.

Parbatya's example also doesn't entail equal percentage curve IMO (by definition) although he was able to yield y=10% for x=40%.

Wait till the evening when i have time, i will get this done if you are not able to solve it till then.