Not sure I can give you a lot of tips here, I always sharpen with the PS window at 50%. I can get results that are fine to my tastes, you would probably find what works best for you by trial and error. Take a sample print and try different sharpening parameters, printing all of them until you find the grail.
There's indeed a software that sharpens according to the output size. That saves me a lot of work as I only need to sharpen the master file and don't need to rework the image according to the output : Qimage http://www.ddisoftware.com/qimage/
It works great, but they need to work on their interface. It takes a while to get used to.