To get the values above and below I suggest the following:

Below
{=3DSUM(--(A1{0,9,19,29})*--(A1<{10,20,30,40})*{0,10,20,30,40})}

This will return say 0 if a1 is between 0-9 , 10 if it is between 10-
19 etc etc

Above
{=3DSUM(--(A1{0,9,19,29})*--(A1<{10,20,30,40})*{10,20,30,40,50})}

The result of these formulas would be the lower and upper nodes in your
interpolation formula.


The linear interp function for the case above would be something like
this:

interp value =3D X0+ (X0 - X1)/(1.2-1.1) * (1.15-1.1)

where X is the data column.


They are equivalent to a series of nested IF=B4s with ANDS included,
albeit more clean and readble IMHO.

Hope this helps