Table 2 Values Gives A Third Value

I have a table showing data for Pipe Loop dimensions for thermal expansion.
In the first column, I have the pipe sizes. The rows contains deflection data for each size.
In the last row, i have data for the dimensioning of the loop, say a value "a".

In the spreadsheet form, the user inserts the pipe size, say 8", the temperatures, and some other data.

Based on the input data, excel will calculate the deflection of the pipe. Say 2".

So i want excel to lookup the size of the pipe in the first column, 8", and then lookup in the row containing the 8" size, the calculated deflection, 2". The result would be in column x. Going down this column, I want excel to give me the value of "a" in the last row.

I hope i'm making myself clear. I'd be much easier if i could sketch it.

I have been trying combinations with Hlookup, Vlookup, Row, ... but i'm stuck...
My idea was to have excel determine the row in which the first data, the pipe size, is located, and then do a lookup in that row for the next data, the deflection. And have excel retrieve the third number, a, located in the same column as the deflection, in the last row.