What % viewing to assess print / screen sharpness?

I'm very grateful to everyone for all the advice and help with this. It seems the assessment of print sharpness from the screen image is not straight forward! However, I now have some very useful practical tips which are helping a great deal, and also a much better understanding of the theoretical aspects of the subject (many thanks Apaflo!).

Yet again a DPR forum has shown just how generous folk are with their time & help, so my thanks to everyone!
Phil