Here is the thing, when I try to judge the effect of the radius slider visually (with or without option key) it looks to my eye that running the radius up makes the image more crisp en lowering the radius smoothens the image. So I am inclined to use the control the other way round. This confuses me.
If you don't alter the Amount, yes, increasing the radius slider will give the appearance of more sharpening in obvious edges but at the risk of over sharpening. Reducing the radius concentrates the sharpening to the high frequencies and with not adjustment to the Amount ends up looking like less sharpening.
In practice all three slider; Amount, Radius & Detail need to be adjusted properly to get the optimal sharpening without introducing additional problems or artifacts...Edge Masking (even a small amount) can help reduce the sharpening applied to large surface areas particularly where noise would be visible.
But, the other critical sharpening slider setting is the Luminance Noise Reduction. Noise reduction comes into play when you are applying strong amounts with a higher detail setting. Adding a touch of noise reduction allows you to go into the amount and use higher detail settings.
And, of course, this is all dependent on the image source and content and ISO.
The other thing to remember is that for ACR 6.x, the Adjustment Brush can also play a role...the current behavior of minus sharpening is to add to the edge mask when below zero and above -50. So, if all other settings are right for an image but some areas are showing too much amount, paining in a minus sharpening can have the effect of adding to the edge mask-note, this works even when you don't have an edge mask set in the Detail panel. You can also use the Auto Mask function of the Adjustment brush to localize the minus sharpening by tone and color.
Your basic approach to setting up some general presets is fine...when it comes down to "perfecting an image" you are right that radius is an important factor, but so are all the other things I mention above. Without actually seeing an image, I couldn't tell you whether .8 would be better/worse than .7 or .9. I tend to go lower than .8 for high frequency edged images but 1.4 seems fine for people and faces...a radius of 1.0 really isn't bad for either edge types...which is why Thomas settled on that as the Default.
