I'd say it depends of the track mostly and then on the turn. On Blackwood with FOX for example, I brake somewhere between 75 and 50 at 1st turn and somewhere between start of curb and 50 at end of the back straight. For the rest of the track I go by feel. Like I couldn't tell you where I lift off for the esses and where I brake for the turn after 2nd split.
Same applies for tracks like Kyoto or Westhill where I find the markers are too far off the track, so I don't bother looking at all and just brake when I think applies. I'd say after 3 to 5 laps I'm good to get it pretty much the same each lap.
On some tracks and turns I use some brake points that are not actual braking markers. A change in the color of tarmac or a spot in grass or a wall, sometimes a curb if it's well placed, but somehow I'd say I tend to use the curbs more as "lift off points" rather than braking points.