In training you should never fail on any lift unless it is a test day.
A good calculator for max rep sets is 2.5% per rep...example, a max triple is about 92.5 %
So, as you can see, a set of 5 with 75% is a good work set, where if you were doing a max 5, you would be around 87-90%
AS for your rep maxes, everything is relative, and you may not have the explosiveness yet to guage your 1RM well. So If you are just starting a new program, pick solid conservative numbers. I have never done the 5/3/1 so I couldnt give any advice directly to it.
Much of my work is based on some Sheiko, and some Hatfield and my own ideas worked in. I find that 5 sets of 3 at 80% is a good work day. Much of my numbers run off of that day, which I got from a Sheiko program.
If you have anymore questions feel free to PM me or check out my training log