HELP! need a lesson on referencing and error checking two cells

So here is the problem...I have created a spreadsheet that does all my calculations for work....2 specific outs are "actual magnetic spacing" (above and below) and "magnetic spacing required".

Problem 1: The magnetic spacing required uses an index formula / indexes a table in a different sheet....In order for the indexing to work properly I had to alter the table and combine both above and below values into a single cell...ie A29,B15 (this is how my output appears)....So I want these values to reference the actual magnetic spacing, and kick out an error message if there is not enough. Actual magnetic spacing is calculated in 2 different cells, 1 for above and 1 for below...now you can see where the problem is.

I have attached to document so you all can put this into perspective.

* I work in the oil field in case your wondering what all this is