What % viewing to assess print / screen sharpness?

I've been given to understand that viewing the output of a 21MP FF camera on a computer screen @ 100% does not reflect the sharpness you’d get in a print.
What % viewing is best to assess print sharpness of a 21MP file for common print sizes such as
5x7 (inches), A4, A3 and A2?

Any help would be greatly appreciated!
Many thanks

Phil