1. Build your own ruler
Fix only the two ends. 1 is barely aroused — early, quiet, not yet much of anything. 10 is the point past which ejaculation can no longer be delayed: physiologically, ejaculation runs in two phases, and once semen has entered the posterior urethra its movement becomes inevitable. Everything between 1 and 10 is deliberately left blank — you fill it in by watching, and the numbers only have to mean something to you. Nobody else's 7 applies to your body.
If a numbered self-rating sounds homemade, it is — and that's fine. Sex research labs use simple subjective arousal ratings all the time (Likert scales, single items, dials across 132 studies in one meta-analysis), as convenient self-report, not as a lab-certified instrument. The validated questionnaires out there look quite different — one runs to 54 descriptors. A 1–10 ruler is a convenience tool you keep at home, not a standardized measurement device, and it doesn't need to be one to be useful.