How far away do you sit from a 27" inch Imac screen? Just to be safe

Arms length distance, corrective eyewear as needed, and controlling direct, or incident (window) lighting. Take several breaks from the computer to improve bloodflow in your legs, and force your eyes to adjust to longer focal distances.

OSHA's guidelines for workstation distances are between 20 and 40 inches to the surface of the screen.

Apple has an article on Mac ergonomics which provides some guidelines that may help:

Ergonomics - Apple Support

Here are some relevant excerpts from the article that may help answer your questions.

Change your screen's height

Raise or lower your screen, so that the area you’re looking at aligns with your brow or slightly below it. If your screen is too low, raise it with a stand.

Adjust your screen's distance

Move your screen a comfortable distance away from your eyes, between 20-30 inches (50-75 cm). Adjust the screen’s angle to be comfortable for you.

Center your screen

If you use one device or display, center it in front of your body. If you use multiple screens and spend more time with one, center your main display. If you use them equally, center them together.

In my case I found that my normal reading glasses were not suitable for my computer use. I now have a separate set of computer glasses which have the focal point set to my normal iMac viewing distance. My optometrist added anti-reflective coatings and a blue light blocking coating which is supposed to help reduce eye strain when using computers.

Normal arm length.

Safe from what?

Safe from what?

As far as I am concerned, proper posture, and a comfortable reading distance are the primary considerations.

What other concerns do you have?