Range Factor is very dependent on a lot of things that have little to do with your player's defensive skills, mostly your pitchers' ratings (handedness, velocity, GB/FB). It's also interdependent; if you play a bad defensive OF your IF range factors will increase a bit.

I ignore range factor. When I want to look at a defensive stat it's +/- or errors.

As already stated, RF is a result of many factors outside of the player's own abilities.
Also, the game distributes fielding chances according to position.
So to my mind, RF is only useful when comparing players who play the same position... and then, I only care if someone has a notably LOW RF.

Why? When I started playing HBD, I had a team without a true CF, so I put a utility player/LF out there. In my inexperience, I figured a low 70s Range wouldn't hurt too badly. Until a friend pointed out my guy had the lowest RF of any CF in the world. I figured that meant too many balls were dropping for hits.

Wouldn't those show up as - plays then damag?

I didn't think at the time to correlate it to minus plays. I took it more as the fact that he wasn't making any plays.

There is a correlation between range RATING and +/- plays. Range FACTOR as a statistic has more to do with your pitchers' ground ball (GB) / fly ball (FB) rating than it does with range rating of a position player. This is shown by the fact that the player who has the highest range factor at a certain position rarely has the highest range rating at that position.

For me RF is a useless statistic that I ignore.