Hi @b_mitchell , I think a couple of value lookups, a joiner and a math formula can do it, if my understanding is correct
Value lookup can be set to return “exact or larger”, or “exact or smaller”, so do it both ways and then for each value of N you have A and B in both directions from N, if N doesn’t exactly match A. Join together the results, and you then have the following table:
After this, you can do a math formula which finds the point between B and B(right) which is proportional to the point where N lies between A and A(right)
(assuming my calc is correct, but if not, I think all the pieces are here so just need to fix it, and I need more coffee 
)
( $N$ / ($A$ + $A(right)$ ) ) * ( $B(right)$ -$B$ ) + $B$
proportional vlookup.knwf (84.4 KB)
[edit: if it is possible for N to be “off the scale” you will have to determine what you want to occur in that situation]