Depends on what kind of bonsai you want. are happy with the diameter of the trunk at soil level? Are you happy with the surface roots? Chopping g the trunk wil slow development of those to a crawl
Fwiw. The thin er the trunk the lower you chop. That increases the perceived scale of the tree. Tall thin trunks indicate young trees. Thick. Stout trees covey more age.
along with the trunk diametrically you have to consider how tall the end tree will be. The top above the chopped trunk will grow back in and built with subsequent chops.
The initial chop is a scene setter for the rest of the tree. We could tell you where to chop but can’t do the follow through
Mostly it’s up to you