The magneto speed is definatley the way to go if you cant afford a Labrador. It doesn't always effect he POA, It depends on the barrel and caliber. Guys mount them on their barrels during ELR competitions to read every shot to identify if a miss was due to a velocity variation. On my .308 and 6.5CM, they effect POA and group size even when they are mount ed from the picatinny rail and not the barrel. However, my chrono is only used to give me the velocity on a given day so I can have my ballistic calculator prove accurate DOPE
However, with the OCW method that I sent you, you don't need a chrono to identify accuracy nodes. You are looking for similar POA among groups. Once you see multiple groups with a similar POA ( regardless of group size), then that is where your accuracy node (velocity plateau) is. There are usually at least one high and one low and are spread over a .5gr depending on the powder. I three rounds in .3gr increments. Once I have identified where the nodes are, I load in .2 grain increments on both sides of those groups to try to identify the center of the node. Once you have found the ends of the node, you pick a charge in the middle. If your rtesting is accurate, you can load .1 and sometimes .2grs on either side of this charge without a change in velocity (which means without a change in POA). That's why you will see world class shooters load on a charge master with a +/_.1gr accuracy because it doesn't effect the velocity or POA.
Once I have found a load using the OCW method, I'll run it over the chrono to verify that the SD/ES numbers are good.