Table Lookup/Index Match Help

I have a table with air volume in a row across the top, load down the left-most column, and vane position data corresponding to air volume and load in the middle. I would like to be able to specify an air volume (top row) and a vane position (within the table) and have a formula use the table to determine the corresponding load (left column). For both the air volume and vane position, I don't want exact matches but the nearest lower value. I can't quite get my head around how to use the Index Match functions to do this. I know how to use it to get a vane position value from the table if the air volume and load are known, but not how to do the reverse. As an example see the attached spreadsheet where I'm looking for the formula to put in cell B21 so that for an air volume of 13.8 and vane position of 57, it will return a load value of 725. Please help!

Put this formula in B21:

=INDEX(B3:B16,MATCH(B20,INDEX(C3:K16,0,MATCH(B19,C2:K2))))

Hope this helps.

Pete