There are some guidelines as to the % of total mill volume occupied by ore+grinding media+water. And this generally depends on the type of mill you are using (i.e. ball, rod, SAG, AG, UFG, etc…), although there are some overlaps between. Key is the proportion of grinding media and ore within that % of total mill volume occupied by ore+grinding media+water.

There are some models used by mineral processing professionals that can be used to estimate all these percentages. Of course they all need reliable input so you can get reliable outputs, as garbage in garbage out, hence potential adverse impact on the operation of your mill and productivity.

It is the responsibility of the plant metallurgist to set these as target for the mill operators / controllers to operate the mills.

We measure the charge level itself in overflow ball mills. We grind the mill out every few months and measure the % full of balls. We use this to correct the charge level that was maintained to a power draw setpoint on a day to day basis.

We find that the packing factor of the balls goes up over time, reducing the % full needed to maintain a given power setpoint. This has to be rectified every few months with a higher than normal charge of new balls to bring % full back to specification.

The 60% "packing factor" for the balls is pretty close to our average for a seasoned charge.

Pulp density is maintained at 40 to 50 % solids and ball charge volume factor is normally kept at 18.5%.

I have to disagree with you... it is not "cast in concrete".
Depends on the ore and mill type and for what goal or purpose.. 

What if it is a dry milling operation?
Sag mills in the gold and copper operations would vary from 6%- 15% and are normally grate discharge types in most cases the densities of the slurry in the mill is over 65% and that goes for ball mills also.
Off course you have a upper limit or the balls will "float" out.
Higher densities allow better grind and increase residence time in the mill.
I don't know why we'd be looking at these numbers in particular but the best will be to know the volume of the mill then between liners and lifters to the point of hydraulic overflow in the case of wet mill of the overflow type, This is followed by the ball load volume measured in reference points in the mill and the weight measured before addition where possible- this gives the void volume also and when then looking at the relative density, you can deduce the ore tonnage, ore volume, dry weight , water volume and weight as well and have all your questions answered.
operators would look at power draw as mentioned in another reply.
Modern mills operate with load cells and mill optimizer's which synchronize the mill weight, power draw, ore feed and weight with density control.
Where optimizers are not used, the rule of thumb for a overflow ball mill is a ball load up to about 12 inches below the trunnion discharge- this allows bed expansion by the slurry filling the voids and lifting the steel balls.