Lookup

I am trying to update a spreadsheet based on the values in the attached table. I am new to the more involved formula's on Excel, and my brain is fried trying all the different possibilities. None seem to be doing what I need which is to select the value based on the column and row header (as identified in my spreadsheet that I am updating).

For example, the item to update will have a step of 4, and item is R3. the value should be 30.48.

How can I format the formula so that both the column and row are variables in selecting the value?

Any help is greatly appreciated.

Thanks,
Paul