Hi Rob
There are no hard and fast rules for the unsharp mask tool. It is very much individual taste. Settings can change for different types of photos (landscape, portrait, etc.) but some people find a setting they like and use it as a general sharpening setting. I tweak each photo a little. Try starting with a Radius of 2 and threshold of 4 and amount 75%. Then try varying the amount. Keep the preview box ticked and you will see what effect it has on your photo. You can zoom in on the preview box too, to let you get a better view. I rarely adjust the radius, but sometimes play with the threshold. Quite often we are advised only to apply the unsharp mask last, after making all other adjustments like curves, levels, colour balance, etc. However I recently read a great tutorial which advises applying some unsharp to begin with and then as a final adjustment too, which I find a much better technique. You will also find that different amounts of sharpening may be required for prints.
Here is a link to some info.
http://www.cambridgeincolour.com/tutorials/unsharp-mask.htmUltimately it is all about playing with the settings and finding what you prefer.
Rgds
don