YOU don't measure chamber pressure at all, unless you are a very serious amateur ballistician. It takes a good deal of equipment, preparation, and operation. As said, go by the book(s), the powder and bullet companies have the equipment and the specialists to be sure that the loads they publish are to industry standards.
Eyeball guesstimation of chamber pressures are not very reliable. One of the old time manuals, back when handloaders were considered to be competent adults, said to increase loads until ONE of several indicators of high pressure was seen, and then to reduce that load by 6%. But that amount of reduction was too much for the hotrodders, who would back off half a grain to just barely avoid glaringly obvious signs of excess pressre, even though it was still there; so the literature quit describing the procedure.
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