Scale it to the size you will be printing it at.
Set the screen view zoom to 1 to 1 (100%)
Flexi is not known for its accurate screen rendering (maybe in the cloud it has been improved) but you should be able to see how pixelated it will print. Look for compression noise & blocky solid colors.
If it looks bad on the screen it will most likely print badly. It may smooth out some depending on what you are printing on and your RIP settings.
100 dpi at production size is a pretty good general rule of thumb for most wide format prints.
wayne k
guam usa