I've made a little test with an unsharp pic of a feather on the pavement. Four layer copies, all set to mode "value", USM-amount is always 5 (in words: five!), radius starts with 0.5 and doubles, opacity halves with every layer upwards.
Looking around in the internet, this seems to be the standard procedure; up to now I didn't find any discussions on the radius and so on. The fine tuning is done by masking and varying the opacity of the layer group. I didn't use a mask but I merged down the USM-layers, so (instead of a layer group) I have one sharpening layer to play with.
Not bad at all. You can even see little drops of water on the feather (it was a rainy day). But time consuming!