Switching from a rigid to a suspension fork – a2c length calculation

Finally getting around to putting a suspension fork on a bike that currently runs a rigid. The new fork can have the travel and therefore the a2c reduced so I can fine tune the a2c.

But what a2c difference between the two is ideal do you think? You’d think the old rigid fork should be equal to the suspension fork’s a2c minus the sag but I’m thinking maybe a bit longer would be right to take into account the length it would be reduced to in a big hit.

Thoughts?