...with all these catered for should I be looking at something around 75% or say 50%?
With "all those catered for", I have no problem thinking of 50%-75% as a good healthy range. Whether it should be closer to 50% or 75% will depend on the metrics for "all those" and a couple others.
I've worked with systems that had no problems at 95+%. But I have no idea what your values are for "optimum performance / cost". Nor do I have any idea what performance tuning has been done on your systems, nor what your workload profiles are, nor the volitility of your database, nor the prevailing access methods, nor any factors such as journaling/commitment control, nor...
Given what's known, I don't think there's any good way to get closer than a guess of "50%-75%" for "optimum performance / cost". It's as good a guess as any.
First thing to do is find out what is take up space on the disk. The more we have, the more we collect data. Sometimes a good cleanup can reduce disk utilization by 10 to 30%.
Next you need to set policies on file retention.
Now that your system is clean, if you are at 80%, you should have ordered more disk and be waiting to put it in.
If you are running below 70% you should be OK.