The most common and most likely answer to this crossword clue is the 5 plus 8 letter word READY RECKONER
I've seen this crossword clue in The Daily Telegraph
R
E
A
D
Y
R
E
C
K
O
N
E
R
More clues leading to the result READY RECKONER
Buddy explains!
The straight part
We know that "handbook for working out values" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case handbook for working out values = ready reckoner.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.