Two-way table lookup with interpolation

Hi, I'm plotting charts and want to avoid making massive excel tables to do a lookup. I've got two input parameters, one for A2:A28 and one for B1:M1. I've played around with LINEST but can't get it to work. I've attached the database. As an example, let's say my input for A2:A28 is 7.8 and my input for B1:M1 is 695, i need the output to say 10931.73075. Please give me some knowledge, I've got a few dozen more charts I'm going to have to implement this to and it will help me a ton.

Thanks!