Programmatically set custom order (reorder) in orderable collection via Local/REST API

[thisanimus]

In Payload, I see that custom ordering within a collection is accomplished via fractional indexing. This order is typically changed via drag and drop, but I find I need to set the initial custom order programmatically once.

Questions:

1. Is there a way to set the fractional index based on a legacy field (number) I previously used to manage ordering?

2. On collection item creation or update via REST or Local API, is there a way to define the order?