Take off and landing trim settings strongly depend on how heavy the aircraft is. It can make enough of a difference that many FMC (flight management computers) in modern airliners have a function where you input your fuel load, ramp weight and desired take-off profile and it will calculate the correct trim for you.
The thing is that while you want some upwards trim to help with take-off, setting too much of it will degrade your acceleration and setting too little will mean you'll have to use greater force on the controls.
Of course, in our WWII warbirds there's no way to really get it super-precise, so it's just a matter of feel.
I generally prefer to trim neutral for take-off because the aircraft in the sim are tail-draggers. This lets me get the tail up faster (which also increases acceleration) and then i add the upwards trim once i'm really rolling on the take-off run.
For landing i first deploy gear and flaps and then add trim until i need less of an effort on the stick to keep the aircraft level. I find that too little trim means i have to pull back a lot and i lose precision, but having the aircraft fly with just the right amount of trim is not helping me either: if it maintains the correct glide angle on its own while i don't move the stick at all i find that i don't have enough tactile feedback , so i trim it slightly nose-heavy compared to what the ideal trim for the approach would be.
In any case, since absolute precision is impossible most of it is trial and error along with personal taste in how you want to fly.
As for Orpheus' post, trim essentially means "the plane will fly straight and level at X airspeed". The slightest amount of deviating from that and you need to re-trim.
If you add power it will climb, maneuvers cause drag and you bleed speed so the optimum trim changes again, and so on.
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