Trunk chopping usually isn't a one-shot process - generally, there are multiple trunk chops which occur higher and higher up the trunk. This produces a more gradual taper as opposed to a drastic, sudden, and single change in trunk thickness (as you mentioned).

Usually, you wait until the trunk is at the desired thickness before chopping. So, if you want a trunk at least an inch and a half thick, wait until the trunk is at least an inch and a half thick.

- Audric