Firstly you need to understand that the airflow over your main will be adjusted and changed by your Genoa, so always trim your Genoa first.
Then the angle of attack and flow will come from your Genoa over you mainsail and off the back of the main.
Traditionally most cruising yachts have only telltales on the trailing edge of the mainsail, this is because the luff (front edge) is mostly adjustable by outhaul, something many cruiser don’t have.
Personally I like to add others but will discuss that later.
For now, the point of the telltales on the leach of your sail, the trailing edge, is to show that the air is flowing off the sail cleanly and not creating turbulence on either side.
If your sail is trimmed too tightly the telltales will be sucked back over to the leeward side.
If your sail is trimmed too loosely the telltales will be pushed toward the windward side.
The ideal trim is to trim up until you have them being sucked onto the Leeward side and then ease off until they just become stable in flying straight out behind.
I do this with a relatively tight mainsheet, using the traveller to adjust the angle of attack and focussing on the lower telltales. Then I use the mainsheet to introduce twist and lesson the angle of attack for the higher up telltales.
The wind higher up is usually at a higher speed and therefore also at a steeper angle than the wind closer to the deck. This is why this technique works are reduces playing around with the traveller all day.