The lowest useful magnification is usually limited by the exit pupil of the eyepiece. This is the diameter of the image produced by the eyepiece/scope combination. Now the best dilation of eyes is about 7mm in young people, this lowers to about 5mm by 50 years so if you don't want to lose any light then the lowest effective magnification would give an eye relief between these values.
How to calculate is discussed in
this thread on CN.
So using this formula Exit pupil = Focal length eyepiece/ Focal length scope a 30mm eyepiece would give you a 6mm exit pupil with your f5 scope.
Hope this helps.