why does your calculator dosnt contain the logarithm for a costum value? :)
example: 2.17 = 5.1^x ...-> x = Log_5.1 (2.17) ...-> x = 0.48
probably because, like most modern graphing calculators (excluding the TiNspire )
its difficult to code also
you can use the change of base formula in order to add custome values.