To do this one to death…

If you use the TREND formula using all the data, sometimes the results will be significantly off because the relationship is not a straight line:

using the chart, you'd expect a volume of roughly 2.2 for level 35.

Using a straight line through all the points you get 2.55 (dotted line on chart)

Formula:

=TREND(D2

12,C2:C12,F3,FALSE)

You can get better results using the small straight line between the points directly before and after 35, this gives you 2.19

Formula:

=TREND(INDEX(D2

12,MATCH(F3,C2:C12)):INDEX(D2

12,MATCH(F3,C2:C12)+1),INDEX(C2:C12,MATCH(F3,C2:C12)):INDEX(C2:C12,MATCH(F3,C2:C12)+1),F3)

or a bit shorter:

=TREND(OFFSET(C2:C12,MATCH(F3,C2:C12)-1,1,2),OFFSET(C2:C12,MATCH(F3,C2:C12)-1,0,2),F3)

You'll very likely get even better results using the fifth order polynomial (the what?) which is a good fit, which, surprisingly has a simpler (well, shorter) formula, and this gives you 2.18

Formula:

=SUMPRODUCT(LINEST(D2

12,C2:C12^{1,2,3,4,5},FALSE),F3^{5,4,3,2,1,0})

See attached workbook where you can alter the value in cell F3 (light green).

Dead horse flogged.