Which level to start with

If you are genuinely unsure which level a given task calls for, medium is a reasonable place to start for almost anything, since it is calibrated as the everyday default rather than an edge case in either direction.

Adjusting up or down based on how the first response actually looks is faster in practice than trying to judge the right level purely from reading the request before seeing anything tCode does with it.

Over time, a rough sense of which of your typical requests need which level tends to develop naturally, at which point the choice stops requiring conscious thought at all.

There is no penalty for guessing wrong in either direction; setting medium for a task that turns out to have wanted high effort just means the result may need one more round of correction than it would have otherwise, not a failed turn or a wasted one.

For most people, the practical pattern that emerges is leaving effort at medium as a standing default and only reaching for /effort high deliberately, for the specific request in front of them, rather than reconsidering the setting on every single message.