interesting interpolation

G'Day guys,
need some help with this code. am trying to do a graphical interpolation by VB.

I have attached a simple example of what I need. The values column need to be filled!

Any idea? So far I can get the initial cells. For example for the first two values I can get out 2 and 3 in the A axis and 4 and 5 in the B axis. But keep getting errors everytime I try and get the four cells which the above 4 values intersect at (as soon as I can get those intersecting cells it is simple maths, no problems).

Thanks in advance for any ideas.