ecu fuel trim

[totta Crolla]

Can anyone help me with interpolated fueling on a programmable ecu ?

I understand the concept of interpolation but I'm a bit stuck on where
to add or remove fuel.
Say I want a particular area of the map to be 10% richer do I add
2.5% to each of the four adjoining cells in that area or would I add
more fuel to fewer cells ?
Is there another way ?

Thanks