Block / buffer size. Make it as low as possible without any audio issues for the best (lowest) latency. 128 is really good, anything up to 512 'should be fine'.

Determine actual latency.

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

Determine reported latency error.

1. [Reaper] Delete second marker.
2. [Reaper] Track 2: Delete item.
3. [Reaper] Preferences > Audio > Recording > User audio driver reported latency, check.
4. [Reaper] Record.
5. [Reaper] Track 2: Put a marker (M) at the same point.
6. [Reaper] Double click in the ruler (top) in the space between the two markers (creates a time selection).
7. [Reaper] Right click in the ruler (top) in the space between the two markers > Samples.
8. [Reaper] Bottom, Selection: [from] [to] [error number of samples].

Set correct latency.

1. [Reaper] Delete second marker.
2. [Reaper] Track 2: Delete item.
3. [Reaper] Preferences > Audio > Recording > User audio driver reported latency, check.
4. [Reaper] Preferences > Audio > Recording > Input manual offset, set samples to [error number of samples]

Check it.

1. [Reaper] Record.
2. [Reaper] Waveforms should match.