Hi guys,

I hope this is in the right spot.

I have a set of data and I'm trying to return a value where y = 30. This is really easy for a linear or polynomial set but this data is plotted on a semi-logarithmic chart with straight lines in between points. I have attached an Excel file, because it's a bit difficult to explain.

You can see in both graphs, using either INDEX/LINEST or FORECAST gives slightly the wrong result. Both formulas look only at the data point above and the data point below 30 (Y) but I'm getting 4.06 and 3.95. I can see that X should equal about 3.65.

As I said; the spreadsheet should better illustrate what I'm getting at.

Can anyone help?

Many thanks all,

Mick

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