Start pressure and case weighing value are the two variables you can tweak so that a given loading matches up with the QL prediction.
By using several different velocity data points over different charge weights, you can then tweak the case weighing value number and the start pressure charge value to match....ie x grains of x powder equals x velocity....there will be only one combo of the above two variables that will match up over several different data points. So the more velocity data points you have, the better your "match" will be.
Don't change COAL, bullet, primer, temp....just charge weights during your testing.
Remember to take into account bore condition...as a "fouled" barrel will shoot faster then a freshly cleaned one. Most of my rifles need 2-4 "fouling" shots for the velocities to settle down.
I would also ensure your exact case volume in known (by measuring water case capacity to case overflow IAW the QL guide), and to actually measure crown to bolt face barrel length. These two important details can throw you astray.
Temperature, depending on the powder, can be in important variable to record during your testing...generally not from one range session, but if trying to use data recorded from several different range session to match up "your rifle/load" combo to QL. RL25 loaded ammo on the dash with the heater on the way to the range, vs sitting in the ammo box in the back of the truck on the way to the range....not good for testing