I've practiced with the front brake, working up gradually to hearing the tire just start to announce it was about to be sliding. I let off immediately. But IF a guy could hold it right there, that would be maximum stoppage the tire could provide.. And if the pucker factor allowed the brain to tell the foot to begin carefully applying rear, that would be the best the bike is capable of.
In the
F.W.I.W. column
Nothing is gonna avoid a T-Bone into the side of a car that is two seconds from impact when you see it.

A few years ago I started to go to dinner with some very experienced riders - many of them have been road racers, and I found out that most of them never use the rear brake!  It took me a while to understand this concept and appreciate why they rely only on the front brake.......then I learned in a way that makes it hard to forget the lesson!

My ST1100 had a rear brake that was not easy to modulate at very light pedal pressure.......it was very easy to lock up the rear wheel when braking very hard with the front brakes.  Under most rides when I needed to brake hard to miss a deer or some crap in the road - it just caused a moment of slide and I could let take some pressure off the pedal without any big issue.

One summer when I had that big bike on the Tail of the Dragon I was braking hard into a corner, and the rear wheel locked up and the back of the bike instantly headed to the outside of the corner....I had to let off both brakes, stand the bike upright, and then get on the brakes  as hard as I could....I no longer had enough room to get the bike leaned back over and into the corner - so I had to hope I could get stopped before I ran out of pavement.  Thankfully I went onto the shoulder at a walking speed and there was room for me to get fully stopped before the ditch.  Thankfully I get these kind of reminders now and then to keep me from doing something really stupid......and I have learned to ride 700 pound bikes with a bit more respect!