Calculated time required to replace pair in hands-on session (8)
I believe the answer is:
tactical
'calculated' is the definition.
(thesaurus)
I don't understand how the rest of the clue works.
(Other definitions for tactical that I've seen before include "Clever - prudent" , "Relating to manoeuvres in the presence of the enemy" , "Of carefully planned actions to achieve ends" , "Done to support skilful planning" , "Strategic" .)