preliminiary implementation for the early heating finish.

Might be replaced by something more clever, e.g. by erik, and does not yet support the second extruder or the bed.
its kind of not so cool, because you need 6 more ints.
Maybe isheating() should use the degrees directly, as it is not used in time-critical anyways.
Then it would be much easier. to have the offsets without additional variables.

Marlin/Configuration.h

 // if CooldownNoWait is defined M109 will not wait for the cooldown to finish
 #define CooldownNoWait true
 
+// Heating is finished if a temperature close to this degree shift is reached
+#define HEATING_EARLY_FINISH_DEG_OFFSET 1 //Degree
 // PID settings:
 // Uncomment the following line to enable PID support.
   

Marlin/temperature.cpp

 //===========================================================================
 int target_raw[3] = {0, 0, 0};
 int current_raw[3] = {0, 0, 0};
+int heatingtarget_raw[3]= {0, 0, 0};
+
 
 #ifdef PIDTEMP
   

Marlin/temperature.h

 float analog2temp(int raw);
 float analog2tempBed(int raw);
 extern int target_raw[3];  
+extern int heatingtarget_raw[3];
 extern int current_raw[3];
 extern float Kp,Ki,Kd,Kc;
 
 FORCE_INLINE void setTargetHotend0(const float &celsius) 
 {  
   target_raw[TEMPSENSOR_HOTEND_0]=temp2analog(celsius);
+  heatingtarget_raw[TEMPSENSOR_HOTEND_0]=temp2analog(celsius-HEATING_EARLY_FINISH_DEG_OFFSET);
   #ifdef PIDTEMP
     pid_setpoint = celsius;
   #endif //PIDTEMP
 };
 FORCE_INLINE void setTargetBed(const float &celsius)     {  target_raw[TEMPSENSOR_BED     ]=temp2analogBed(celsius);};
 
-FORCE_INLINE bool isHeatingHotend0() {return target_raw[TEMPSENSOR_HOTEND_0] > current_raw[TEMPSENSOR_HOTEND_0];};
+FORCE_INLINE bool isHeatingHotend0() {return heatingtarget_raw[TEMPSENSOR_HOTEND_0] > current_raw[TEMPSENSOR_HOTEND_0];};
 FORCE_INLINE bool isHeatingHotend1() {return target_raw[TEMPSENSOR_HOTEND_1] > current_raw[TEMPSENSOR_HOTEND_1];};
 FORCE_INLINE float isHeatingHotend(uint8_t extruder){  
-  if(extruder == 0) return target_raw[TEMPSENSOR_HOTEND_0] > current_raw[TEMPSENSOR_HOTEND_0];
+  if(extruder == 0) return heatingtarget_raw[TEMPSENSOR_HOTEND_0] > current_raw[TEMPSENSOR_HOTEND_0];
   if(extruder == 1) return target_raw[TEMPSENSOR_HOTEND_1] > current_raw[TEMPSENSOR_HOTEND_1];
 };
 FORCE_INLINE bool isHeatingBed() {return target_raw[TEMPSENSOR_BED] > current_raw[TEMPSENSOR_BED];};