A tacky elastomer, the pressure-sensitive adhesive spontaneously bonds with a solid on contact to form a load-bearing pressure-sensitive adhesive joint (PSAJ) that makes many manufactured goods. While the property called the tack of the PSAJ, measured by the green strength of the joint at low applied stress, is vigilantly monitored for quality assurance, it harbors considerable indefinitude because it increases with applied stress and age to span a continuous spectrum that no finite number of discrete empirical readings can fully cover. Assuming the joint strength to be proportional to adhesive/substrate contact area on the atomic level, and treating the contact process as atomic-scale creep, this work determines from first principles the measured tack of a given PSAJ as a function of applied stress and age. The quantitative structure of the function and its implications are explored. The result is tested with published empirical data and used as a basis for mapping the tack spectrum from discrete readings. It brings settlement of long-standing issues such as ranking PSAJs by their non-corresponding tack readings, explaining the disconcertingly high noise : signal ratios plaguing tack data, buffering the uncertainty and extending the domain of tack measurement. It also shows that the tack and the mature joint strength constitute one smooth continuous spectrum.