Its not written anywhere but if tagged target dies you will recive 1 stack.
Only time when you got less than 20% is if you tag a different target before previous is dead within 2 seconds. which is kinda sucks, but its not a big deal at all.
You can get above 20% most of the time.
If you rely on spam tagging to see stuff (which is not going to make it much worse), try using executioner stance instead.
Target down modifier will also regain more than 10% toughnes and stam most of the times