Measured acceleration (the default) is recorded from the accelerometer. Only in case this fails, you may calculate acceleration from GPS (Lap List -> lap -> Recalculate -> Acceleration). The process to calibrate needs to follow this pattern: while standing still on flat surface in the pits, add your smartphone to the mount. In case you rotated the device, LapTimer will prompt you to confirm your phone is in the final position. Do not confirm until you have fixed the device. Once you confirm, LapTimer will calibrate the accelerometer to compensate gravity and tilt. The step performed on track is compensation of yaw. You should see a roughly zero overall acceleration (center of circle etc) before you start to drive.
In case you start driving without the initial calibration steps finished, LapTimer will not calibrate on track. A typical issue is when mounting he device while driving all track already. This does not work and is discouraged for safety reasons anyway.
For more information, please search the forum. There are lots of threads on this already.