need help with output sharpening on iMac 5k screen

Disclaimer first: I am still struggling with a dedicated processing workflow and maybe I'm doing something wrong or maybe i need to do something very different, but here goes: Love my new iMac and i have noticed that when i open the jpeg in PS for final sharpening, first it opens as 337% on the screen and it looks horrible. So if i open it at 100% it looks better, but it is small to work on for final sharpening. Currently i either take an image from DPP or LR to PS and since it was already sharp, I make any clean-up changes and denies additionally in PS, then I send it back to LR as a tiff, and export it as a jpeg, then open the jpeg in PS where it looks terrible... The other choice Ive recently tried is in PS, i export for web and then open the jpeg and apply USM... then hit save and that jpeg is now sharpened for web... Same problem though, when i open the jpeg on my screen it looks terrible and if i were to sharpen that version the image would be trashed... So what steps am i missing and is there anything specific i can do for my 5K screen?

Thank you, Ann

Go to preferences pick display and then options for scale and change it to one of the smaller sizes this will show the image bigger on your screen once it is sized for the web.