Subtract time it took reaching the evaluation from the press duration. (#208)

Adresses #206

src/main.cpp

@@ -150,8 +150,10 @@ void verifyWakeupLongPress() {
   // Give the user up to 1000ms to start holding the power button, and must hold for SETTINGS.getPowerButtonDuration()
   const auto start = millis();
   bool abort = false;
-  // It takes us some time to wake up from deep sleep, so we need to subtract that from the duration
-  constexpr uint16_t calibration = 29;
+  // Subtract the current time, because inputManager only starts counting the HeldTime from the first update()
+  // This way, we remove the time we already took to reach here from the duration,
+  // assuming the button was held until now from millis()==0 (i.e. device start time).
+  const uint16_t calibration = start;
   const uint16_t calibratedPressDuration =
       (calibration < SETTINGS.getPowerButtonDuration()) ? SETTINGS.getPowerButtonDuration() - calibration : 1;