This is the 65K dollar question. I've struggled with this myself, and have concluded that each rifle will be different, as the "start pressure" captures effects from more than just distance from lands, but neck tension, throat design, lead angle, and tightness of bore......
I have found Quickload to be most accurate when you can load a string of rounds (obviously the more the better i.e. 10 rounds) at varying charges, that are all shot/loaded with all other factors being the same (primer, powder, brass, neck tension, distance from lands, bore condition) and then recorded those 10 velocities over a chrono. If two or three of those charges were near the "max load" for that gun, I find that helpful for the next step.
-(collecting range data)-Load 10 rounds at varying charge weights/steps, with all other factors being the same i.e. COAL, primer, powder, neck tension, case prep on cases, brand of cases, number of firings on cases, bullet. Collect your chrono data, and ensure you correct for chrono distance from muzzle to have true corrected muzzle velocity.
-(initial input setup of Quickload for the range data)- Back at the computer we'll now attempt to calibrate Quickload to your gun. First input the temperature of the cases when they were fired. (see note 1 below)
Once the temp is plugged in, we ensure the COAL, barrel length, and case weighing factor all correspond to your rifle and case. (see note 2 below)
-(Low/Slow load) Now plug in the lowest charge weight, and a nominal start pressure (the ones suggest by QL are excellent suggestions). If the velocity is lower than what the corrected chrono muzzle velocity is, increase the start pressure to make the velocity match in QL. If QL was faster, lower the start pressure. I would use 1000 psi increments. Once you have QL matching with your low/slow load, move to the next step-
-(High/Fast Load) Now input the highest charge weight, with the same start pressure/case weighing factor that you used on the low/slow load. Does the velocity match? If not...here’s where the “magic” from you comes in. You’ll need to adjust the case weighing factor and the start pressure so that QL can accurately predict the velocities at both the high/fast load and the low/slow load.
Use of the following rules of thumb can be helpful. For a given set of conditions:
1. Raising start pressure will raise velocity and peak pressure and vice versa
2. Lowering the case weighing factor will increase velocity and lower peak pressure and vice versa.
-This can take a bit of time. But for your investment of time, you’ll be able to plug in all the load step/increments from your shot string, and QL should be spot on. Additionally you’ll have a great place to start regarding start pressure/case weighing factor for other powders.
If you can’t get the QL prediction to line up on both the slow/low and high/fast load with the same start pressure/case weighing factor, you likely either have a ES with your loads or a chrono problem….don’t ask me how I know:(
I now shoot with my CED M2 at least 25 feet from the muzzle
There’s probably a simpler way to do this…but using this method, I’ve found QL to be very accurate and helpful, and to be a great predictor of velocity potential and where “high pressure” loads might be encountered when desiring to develop another load for a different bullet or powder.
Hopefully this helps someone else. QL is a great tool.
-This may be different than the range temperature, i.e. if your rounds sat on the dash on the way to the range, they were sitting in the hot sun for 1 hour during your session, you allowed the loaded round to sit in the chamber for extended periods prior to firing...etc. I use the ambient range temperature if the round goes off in 15-20 seconds from firing, and I've taken care to keep the rounds in the shade prior to firing. I've also used a small lined lunch box cooler with a bag of ice, not as attempt to keep the rounds "cold" but to keep them at a constant temp prior to firing.
I know some folks will say the effect of temperature on the powder/velocity is negligible, and those opinions are from very qualified folks that I respect, that hasn't been my experience. IME, Temp has effected all powders (even H1000) especially once temps go over 80 degrees F.
Note-(On case weighing factor)
-.40 case weighing factor has proved to be fairly proven for the .338 Edge. I use .36 for a 300 RUM Improved at it's works well. This weighing factor should get you started for your case.