Determine actual latency.
[Reaper] Preferences > Audio > Recording > User audio driver reported latency, uncheck.
[Reaper] Add track, record something.
[Interface] Connect headphones (?) into input.
[Reaper] Add track.
Turn off monitor.
Arm for recording.
Set input.
[Reaper] Play and set levels so track 1 & 2 roughly match.
[Reaper] Record.
[Reaper] Track 1: Put a marker (M) at a distinctive point.
[Reaper] Track 2: Put a marker (M) at the same point.
[Reaper] Double click in the ruler (top) in the space between the two markers (creates a time selection).
[Reaper] Right click in the ruler (top) in the space between the two markers > Samples.
[Reaper] Bottom, Selection: [from] [to] [number of samples].
Determine reported latency error.
Set correct latency.
Check it.