What is the rule of thumb for sizing an expansion vessel for a sealed
heating system?

My application is not a domestic heating system but rather an unvented
cooling system cooling a large industrial machine, but the principal
is the same. The 'boiler' equates to the machine being cooled. the
'radiators' are a fan cooled bank of tubing and the water based
coolant is circulated by a pump.

Given the system volume = V and coefficient of expansion of water per
degree is E and temperature rise is deltaT presumably :

(V(orig) x E x deltaT) - V(orig) = the max increase in volume

But what safety margins are applied in domestic heating systems? Do
plumbers really calculate system volume ?

AWEM