Hi BKGT,

One way to get an exact match is to use an x,y scatter
graph with 2 data series in it.
Assuming your data is in columns A + B and the following
0 4
1 8
2 16
3 27
4 50
5 23
6 16
7 8
8 4

Data series 1 would be
0 4
1 8
2 16
3 27
4 50 i.e. (A1:B5)

And data series 2 would be
4 50
5 23
6 16
7 8
8 4 i.e. (A5:B9)

You can then add a polynomial (4th order) trendline to
each series and check 'show equation on chart'.

The equation for the first series is

y = 0.4167x4 - 2.6667x3 + 7.0833x2 - 0.8333x + 4

And the second series is

y = 1.0833x4 - 27.333x3 + 256.42x2 - 1067.2x + 1688

Now ALL?? you have to do is plug your y value into
each equation (MAX/2 or 25 in this example) and you
can calculate the value of x. Unfortunately my
mathematical skills aren't up to the task at this bit. I can
extract the equations from linear regression trendlines and
use them in formulae but polynomials just leave me
blubbering. Any one else care to step up to the plate and
show how to extract these equations into usable formula
that will reduce the process to
1.Data Input 2.Read off results

Regards
Martin