Hi,

I couldn't follow your exact problem here but I think you are
looking for SLOPE and INTERSECT.

In the equation y = mx +c
m is your gradient (SLOPE in Excel)
c is your y-intersection (INTERSECT in Excel)

So your equation becomes
y = SLOPE(Known Y's,Known X's)x + INTERSECT(Known Y's,Known X's)

with your Known Y's + Known X's being your range of data.

Does that help?
Martin