Yours truly with a more certain follower, one able to calculate quantity (8)
I believe the answer is:
measurer
'follower one able to calculate quantity' is the definition.
I can't judge whether this defines the answer.
'yours truly with a more certain' is the wordplay.
'yours truly' becomes 'me' (the person who set this crossword).
'with' is a charade indicator (letters next to each other).
'more certain' becomes 'surer' ('sure' can be similar in meaning to 'certain').
'me'+'a'+'surer'='MEASURER'
Can you help me to learn more?
(Other definitions for measurer that I've seen before include "One with tape" , "ruler" , "Perhaps tape" , "Maybe tape" .)