ReaInsert auto-adjusts for latency so showing only 1 sample off is probably about right, the best true loopback method is to use a physical cable...
Track 1: Some audio signal such as a single square wave sample. Use a send to send this out of the SC on let's say "Output 3".
Run a cable from physical Output 3 to physical Input 4.
Track 2: Set to record Input 4.
Press record so that Track 1 sends it's audio out and Track 2 receives it. Then measure that delta/latency between the recorded and original. Also...
to get all the latency you probably want to turn off the option in preferences that auto-adjusts for driver reported latency, otherwise you'll only see the unreported latency which on UFX (in my setup anyway) is in 50ish samples range.
^That said, once you've calculated full latency, turn driver reported back on, do the test again, any latency that remains is unreported and you can offset that in record/playback offest settings.
Welcome to the forum btw.