It is not a good practice to become wed to the ruler, just measure when your eyes notice a boner that was previously unrecognizable. Your erection does not have the dramatic flucuations as your flaccid size (but this experience will vary between different genetic endowments), however, the quality of erection can vary quite a bit. States of arousal and relaxation factors will influence the overall expansion of your penis during an erection. Sometimes you can be so unbelieveably excited that blood flow is prematurely shutdown; valves shut down allowing for an extremely rigid member but relaxation factors do not allow optimal expansion.
The result is a smaller than normal corona and a shorter shaft. Neural activity has a tremendous effect and can influence an erection’s variability. You can reproduce a similar result with a full bladder influencing your prostate and relaxation factors in your groin area. The result is a stunted erection. Sometimes you can alleviate these negative influences through hand massage (relaxing the erection and enhancing the blood/pressure effects), other times your just too amped up and you have to get a release to restore normalcy.
Masturbation has no long term influence on your gains and will not affect the activities of a disciplined PE activity. Your to wrapped around the ruler, let it go.
Hang tough, hang long.