If the length fitting result gives the change of more than 1 inch (2,5 cm) it makes significant difference in Swingweight (SW) and Moment of Inertia (MOI). This effect could be partially compensated by adjusting lie angle, but it may not be fully compensated.
The well-known rule of thumb is that ¼” of length trades with 3 grams of the clubhead weight. But it is the general estimation only.
The calculator below shows more precisely how to neutralize the heft changes caused by length fitting. It indicates the weight that should be added to or subtracted from the club head to fully compensate the heft for the length changes and keep:
(i) swingweight unchanged
(ii) MOI unchanged
after the given length change.
update: May 2020