it's really not as simple as just the height of the pivot...… there is no real rule of thumb.
the closer the pivot is to the sear height the easier it is to achieve a light pull but the further your trigger will have to travel to trip the sear. this is the main reason we see set triggers in original guns. the key is to place the pivot where your trigger geometry allows a decent pull without excessive travel. that is why we see the triggers with large areas of steel above the pivot . that area is made large to allow the trigger to be shaped to engage with the sear bar in a geometry that pushes the sear in the most advantageous direction to eliminate as much movement at the contact point as possible. as the pivot location benefits trigger travel (as in a low position) it raises pull weight and movement against the sear bar. because of the distance between the pivot and the sear bar, that arrangement makes a lousy trigger. this geometric problem is what necessitated the almost universal popularity of set triggers in these guns. so basically,... the choice is a set trigger or deal with longer trigger travel or higher trigger pull weight. to achieve a decent pull with a single trigger you have to play many factors against each other. main spring strength has to be balanced against being strong enough for good fast cock speed and a decent trigger pull. as usual it is all a compromising balancing act.
