Instead of:

the timestamp should be the latest timestamp the user agent is able to note in this pipeline (best effort). Typically the time at which the frame is submitted to the OS for display is recommended for this API.

the timestamps should be the time at the end of the "update the rendering" steps, namely the same timestamp used for long task reporting, in section 12.1 of https://html.spec.whatwg.org/multipage/webappapis.html#event-loop-processing-model

Trying to guess when the OS is going to present the frame is fraught with peril and adds no value.