I think different people see light differently on the sights and therefore make allowances differently than others. Due to all these variables, there is no forumla. There may be systems that work - best one is sight black - give it a squirt and the sight is bold and jet black. A wipe of the thumb and it's gathering light as needed in the shadows. Sometimes I forget it. Wide felt pen can also work well - the dry-errase ones are best as they were dull, non refletive - at least the one I had was. Teh permenent felts are shiny and sometimes don't help much, but usually hep some. Accourterments like felt pens, ball screws, spare sights and files along with cameras and a can of sight black are why some of us carry a canvas bag on the left side with the possibles bag on the right. The soot from burning masking tape below the sight (barrel upsidedown of course) will give an incredibly dull black to the sights. I used that on handgun sights, years ago.
I find bright light on a blade makes me shoot higher as the light makes the top of the front sight disappear, so the tendency is to raise it in the notch until one can 'see' it.
Oh yes - sun can play havok even at close ranges of 50yards and closer yet on really small gongs, targets like strings, straws and splitting playing cards even closer yet. The lighting is what made me miss the dog chain at 50yards on Sunday, while both Len and Taylor hit it - that's my story/excuse. I shot second just after the sun birghtened then it went behind a cloud again so Len could hit the chain next. YUP.