Overfilling level depends on if the system has a vented or non vented cap.

A vented cap system can probably be overfilled by the distance between add and full. If the steering wheel is turned with the engine off, fluid might be pushed through the vent and make a mess. ( this is the limiting factor for overfilling )

A non vented system needs an air space to allow for fluid expansion ( heat ) , so I'd call 1/2 of the add to full distance. Late 90's GM front drive cars had a non vented cap and if the system was slightly overfilled, steering turned with engine off, it can push the return hose off the barb. ( as steering is turned, fluid is pumped out of the power cylinder and into the reservoir. )

A quick way to make a vented system is to pull the cap gasket.

At some point the rack may go 100% fail without warning so unless there are $ or time constraints, changing sooner rather than later is better.

Trying to get more miles from the existing one in hopes that the replacement will " last longer" is false economy especially once the cost of additives and fluid is taken into account. If the replacement lasts as long as the original, it isn't going to matter much if it lasts 320 K or 325 K assuming the car is still worth fixing at that point.