The other hand

Wrapping a tape one-handed around something that is not otherwise held still is awkward, so most people steady the shaft with the other hand while they wrap. That hand is doing more than steadying, and where it holds decides how much.

Why a nearby grip changes the reading

Erectile tissue compresses under sustained pressure the same way any soft tissue does, and a hand gripping firmly enough to hold the shaft still applies exactly that kind of pressure. If the grip sits close to the girth site - even an inch or two away - the compression it produces does not stay neatly confined to the skin directly under your fingers. It spreads along the shaft to some degree, and at the midpoint where the tape is reading, tissue that would otherwise be at its resting circumference can be very slightly smaller because of pressure applied nearby rather than at the site itself.

The direction is consistent: a nearby grip pushes the girth reading down, not up. That makes it a systematic error rather than a random one - it does not average out over three readings taken with the same grip, because all three are compressed the same way. Systematic versus random error is the general version of this distinction; a steadying grip near the site is one concrete source of the systematic kind, specific to girth.

Where to hold instead

Hold at the base, below the girth site, rather than anywhere close to the midpoint. The base is far enough from a midshaft reading that grip pressure there has negligible effect on the circumference at the site, and it is also a natural place to steady from since it does not require reaching past the tape.

If the site itself is nearer the base than usual - some protocols and some anatomy put the practical girth site further down the shaft than the textbook midpoint - hold above the site instead, near the glans, rather than below it. The rule is distance from the site, not a fixed anatomical point: keep the steadying hand as far from where the tape sits as the wrap allows.

Grip only as firmly as steadiness requires. A grip firm enough to prevent any movement at all is usually firmer than necessary; enough to keep the shaft from swaying while you bring the tape round is normally sufficient, and firmer than that only adds compression for no benefit.

A check that costs one extra reading

Take a girth reading with your usual steadying grip, then take a second one holding as lightly as you can manage while still keeping the shaft still, at the same site. If the second reading comes in noticeably larger, the first grip was compressing tissue enough to matter, and it is worth adopting the lighter hold, or moving the steadying hand further from the site, for future sessions. If the two agree, grip pressure was not your source of error and the spread you see between readings is coming from somewhere else - skin movement under the tape itself is a separate and equally plausible cause, and worth checking next if grip did not explain it.

What this note does not cover

This is specifically the steadying hand's effect on tissue near the girth site. It does not cover the tape's own friction against the skin during the wrap, which moves skin by a different mechanism and gets its own treatment. It also does not cover the tension specification for the tape itself once it is wrapped - what "snug" means as a stopping point is a question about the tape, not about the hand holding the shaft steady underneath it.

The broader point

Every self-measurement has a hand somewhere that is not the hand doing the reading, and that hand is rarely considered a source of error because it does not touch the instrument. It touches the thing being measured instead, which for a compressible tissue is just as capable of shifting the number. The published studies avoid this problem by using a trained observer whose only job is the reading, with nobody's hand near the site for any other reason - a structural advantage self-measurement cannot fully replicate, but can get most of the way toward by simply choosing where to hold.

This is a technique question, not a rating question, and it stays that way even next to sites that answer a different one. Rate Cock is not measuring a circumference at all, so grip pressure never enters into it - a photograph is scored as it is presented, by an image model working from what the picture shows rather than from anything a hand could compress. A numeric score and a person's read of a photo sit in that same category: neither has a grip to worry about, because neither is taking a physical reading in the first place.