I have heard the best settings for the max and min settings are got from the scene size.
Max radius is 10% of scene size and the min radius is 10% of the max radius.
1) My scene is a three quarter view, so where do i take my measurments from?
2) I am using the options: create> measure tools> distance tools. Is there a set unit i should use like inches or mm? I have read that people use Maya units, How do i change settings to measure in these?
3) Has anyone got any general tips on setting the radius, like how to tweek them to get the best result, like do i make the min radius smaller or larger for more accuracy?
1) same as for full view, radius a is a world scale unit noty screen scale.
2) never change maya unist instead thinkof maya as unitless or ehatever unit you wish... as long as your concequental its okay (maya starts behaving wierd one cyou change teh default unit to something else, so it seasir to scale all other things instead, and offcourse a camera has no scale)
the scene scale suggestion only works as a starting point. If you have small geometric details, specifically like the gaps between the keys on a keyboard, then you have to think about the scene scale in terms of the keyboard's scale, and not per the entire room. And compromise. You can use the min/max Sampling and a low RGBA constrast setting to further smooth out the problem areas, without affecting the render time of the whole frame.