I've failed going from +3 and went to +2.
The guide thingy means going To +3 (From +2) never fails.
Not going from +3 (to +4)
+4 - If item fails, you will go back to +3; medium increase
They need to tell us what the percentages are or else i can't gauge my luck...
Let's just say... going to +5 is very high. I got to +5 on all my equipment and only failed a few times. Though +7 and above might be tricky.