As someone who threaded the needle and got pretty close on numerous occasions this year, it's hard to come up with one set of guidelines. Trying to know the environment is really key, however. Some days it isn't too big of a deal to play close, other days it's a good idea to keep distance. There weren't too many 'holy crap' days in 2015 that I can remember. Storm mode, motion, road network, and things like CAPE and LCL's/dewpoints are all things to think about. If I'm in a gridded road with good terrain and slow storm motions I'm likely to go right up to the tornado. Keeping a watch overhead at the wall cloud/meso and not just at a tornado is also good. I found myself right next to a tornado on May 6th then I looked up and realized I was in a terrible spot, so I backed up.
The real big thing I can suggest is that you have to be able to think ahead and look ahead. Not many people can process maps and multiple escape routes fast while driving and chasing. A lot of people are just not excellent at multi tasking. That's where the bigger risks come in. Luckily I'm quite scatter brained and can multi task pretty well, and I keep a lot of road options in my head at a quick glance. The more I'm in an area too, the better off I am. If I've chased down the road before, I have a pretty photogenic memory and can remember what things are like. Experience definitely counts in this area.
An example is November 7, 2011. I had chased 4/26/2009 (screw that day) in the Wichitas, and eventually went through the Wildlife refuge to stay on the storm that would go on to produce a funnel in Carnegie. I was able to keep up with the storm but barely, and had to break speed limits to do so. On November 7th, the huge conga line of chasers went through the wildlife refuge from OK-54. As soon as I realized we were going 30 MPH, under the speed limit, I knew we'd never keep up with the storm. We punched west through the hook and then took OK-54 north to OK-19 north of the wind farm there on the mountain. We were able to then punch back through the hook and get back in front of the tornado. If I hadn't had previous experience with chasing in that area, I would have likely continued in the conga line and missed more tornadoes or been in a really poor (read: unsafe) spot. Knowing the roads is important and only something you can get with experience.
As far as May 31, 2013, we tried to punch through the RFD and my instincts told me to stop. I wasn't liking what might be on the other side that day knowing the environment was super prime for huge devastating tornadoes and having no visibility.