"...nothing complicated about (it)..."???
Lets see...
IF you have been told to report then wait until the last report, which wiill be at any random level that meets that controller's requirements on that particular flight for that particular instant.
IF you haven't been told to report passing a level, then set immediately
IF the controller has a change of mind and subsequently requires a report then change setting (When? Immediately? Approaching the the guestimated level?), change back after.
IF OCTA then change at the TL. That is, first figure out just what the TL might be where you are since it varies all over the geographic place.
IF other cruising traffic reports at a level that's probably near the one you're passing, how do you assess the conflict?
OR/
Everyone above a single, common TA for the country is on FLs, those below on a QNH/area (regional) QNH.
Which is simpler?