Lookup intermittent value in table and return interpolated value

First, you should probably be clear on the exact interpolation algorithm you need. Especially in multi-D space, there can be multiple interpolation algorithms.

Assuming you want "bilinear" interpolation (the most common 3D interpolation algorithm I see on here), this thread discusses a suitable algorithm: https://www.excelforum.com/excel-gen...-in-excel.html

The main complication in your case is that you want to find Y at a given X and Z. The simplest approach is to do the bilinear interpolation at your best guess for X and Y, then use Solver to (a) set target cell -- the cell calculating Z (b) to a value of -- the desired value for Z (c) by changing -- the value for Y.

No spreadsheet attached.

If I could get the two closest Z values for X (1070 and 959) and their associated Y values (2.2 and 2.4), I can do the math.

That part should be a straightforward INDEX(...,MATCH(...),MATCH(...)) function (illustrated in the other thread). Are you familiar with these functions?