Calculating people may find it useful (5,8)
I believe the answer is:
ready reckoner
This clue might be a double definition.
'calculating people may' is the first definition.
The definition suggests a singular noun which matches the answer.
'it useful' is the second definition.
The definition suggests a singular noun which matches the answer.
'find' is the link.
Can you help me to learn more?
(Other definitions for ready reckoner that I've seen before include "A whiz at mental arithmetic" , "I've done your sums for you" , "Book of calculation aids" , "Printed tables used in standard computations" , "Calculation aid" .)