Just use them as a rough guide, each car may have to brake before, at or after them, it depends how you brake too, trail braking lets you hit the brakes later because you hold them longer into the turn and straight line braking requires braking earlier. But every car is different, just figure out where you need to start braking relative to the cones for each car and then find a point on/off the track that matches that spot.
How well you came out of the last corner plays a roll too, sometimes you have more speed then others when entering the same braking zone and this needs to be compensated for as well.
The most important part of braking is not when to hit them though, it’s where you start to release them, this point needs just as much attention, if not more than the initial braking point, this point shouldn't fluctuate nearly as much if you’re doing it right.