Interpolation in excel with VBA

Hi

I have just started to work on a routine for excel developed with VBA.

One of the sub tasks within the routine is to interpolate between different sets of datapoints. For this task I am using the following calculations:

=FORECAST(NewX,OFFSET(KnownY,MATCH(NewX,KnownX,1)-1,0,2),OFFSET(KnownX,MATCH(NewX,KnownX,1)-1,0,2))

Somehow I have allot of difficulty to implement this into my routine and I hope therefore someone could help me to solve this problem. See the attached file which included the specific part of the routine.

Best regards
John