If your data looks like this and your input is 30,
then Steve's formula will give you 0 as an answer
and Bob's will give 23 as the wrong answer.
Val 145 127 120 160 130 170 160 160 170
Lab 20 21 22 23 24 25 26 27 28
Ref 8 0 8 2 0 10 8 30 30

Input
30
Output
27