In an attempt to KISS (others may add)
There are performance N1 limitation values based on fan aero and mechanical N1 limitation values based on stress. The installer knows the relationships. Depending on the conditions of the day you can operate above the 100% gage reading to achieve maximum permitted thrust (like somebody said 105% or even 117% depnding on the engine specs. However mechanically you will be in do-do if you go much above 105% of permitted redline speed (in the manuals). The first thing that gets you into trouble with N1 is running into unsafe vibration modes e.g. the National airlines DC10 CF6 fan. The second thing that gets you into trouble is rotor disk growth and blade wear and the last thing would be rotor burst (extremely unlikely given that it would take higher than 130% of rediline.)