you cannot reflect more power than applied so the worst case reflection = 1 or in log scale 0dB
The best case is nothing reflected thus a value of 0. (nothing reflected)
But in practice ,in log scale, -15dB limit may be ok, or more negative for more critical applications.
When waves have a speed on T line which depends on dielectric constant and speed of light, it produces natural wavelengths, where 1 wavelength is 360deg. A transmission line is said to be harmonic at multiples of 1/2 wavelengths , like a skipping rope or guitar string.
When odd multiples of a quarter wavelength, impedance is converted in both phase and amplitude. e.g an open cct becomes short and visa versa. are special cases.