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There is no thumb rule for this you can only calculate it based on friction losses in pipe by equivalent length method and for fittings k factors from the various supplier of the fittings ashrae k factors are old they tend to change according to various suppliers.
All suppliers for the fittings have to have the k factors according to prove there manufacturing according to designated area code of practice.
head for primary pump
= chiller friction losses + pipe losses for longest pass + fitting losses
head for secondary pump
= pipe losses for longest pass + fitting losses + last equipment in the longest pass ( FCU or AHU )
there is tow way to calculate the pump head
1- equivalent length method and you may found it in carrier handbook.
2- k factor you will find it in ASHRAE.
the main concept for calculation and sizing
pipe size < = 2" -----> Max velocity = 4 fps
pipe size > 2" --------> max friction losses 4ft wg / 100 ft
there are many programs for pump head calculation like
H-sym (Elite) , Trane pipes , pipe flow expert
