Where to put chalk at the right moment (2,3)
I believe the answer is:
on cue
I believe this is a double definition.
'where to put chalk' is the first definition.
(I've seen this before)
'at the right moment' is the second definition.
(I've seen this before)
(Other definitions for on cue that I've seen before include "At exactly the right moment" , "When signalled" , "When expected" , "One causing death" , "At the correct moment, like efficient actor" .)