Interpolation of electrolytic capacitor's dissipation factor / loss angle
An interpolation (or worse an extrapolation) is only as good as the data points you have, and your reasonable expectation about the order of the function between (or worse outside) those points.

If you don't know the function, there's little point doing an interpolation. 

If you have one point from the datasheet, then a zeroth order (same constant value) is a reasonable guess until you've measured the ESR at several frequencies to characterise the function.

If you have two points, then many people would be happy with a straight line drawn between the two. However, some might prefer a straight line against log frequency, and the more cautious would worry about whether there was a resonance or knee somewhere between them. Until you've measured the ESR at several frequencies to characterise the function, you won't know.