Hey Chris,
I'm not 100% sure what's going to look acceptable on any given Mac pre and post retina.
When I share graphics from my non-retina (1440x900) 2012 MacBook Air the images are 72dpi.
You're generally better off sharing with Keyboard Maestro share to forum mechanism.
And/or using KM-MenuBar ⇢ Edit ⇢ Copy to Image and then pasting into the forum editor.
These methods produce smoother, more professional looking images.
You can select a macro, an action, or a group of actions and get an appropriate image.
When I resize images on the forum I almost always just adjust the visible size via a percentage – I don't actually resize the image itself (expect in rare cases).
You'll note that when you paste an image into the forum editor (or upload one) the right-side wysiwyg display has a 100% – 75% – 50% selector at the bottom of the image.
It that's not good enough you can adjust the percentile in the code section of the editor.
Downsized images are normally light-boxed – i.e. clickable for displaying at full-size.
I haven't reviewed this for a while, but I think it's the most comprehensive discussion on the forum:
Macros for Retina Screenshots on the Forum
-ccs