Quicksilver gives you temporary +50% dodge (and temporary dodge prediction). Temporary as in both effects are cleared when you dodge successfully the next time.
If you have 20% dodge, and drink a quicksilver, then you'll have 70% dodge until your first successful dodge.
Dodge is never guaranteed unless your chance is 100% (you can drink two Quicksilver potions, for example, to guarantee a dodge on your immediate next strike).
Reflex only gives you an extra strike (at "super-priority"), it does not impact your chance to get hit.
If you don't want to waste any dodges, you can do the following:
1. Drink Quicksilver potion.
2. Check dodge prediction against a boss/monster who can hit you. If it's anything but "Dodge", strike a low-damage monster that can still hit you (because it has First Strike, or because you don't 1-shot KO it, or whatever reason); then repeat. (This way you can cycle thru the "non-dodges" and arrive at a dodge.)
3. Once you see "Dodge", drink the Reflex potion and hit the boss; you'll get two hits in without getting any retaliation.