Duel skills clarification?

BST doesn't work like that. The BST that counts towards arena is lvl 40 (no merges) and without any stat increasing skills. If you want to know Caeda's actual BST, check her hero page here. A duel skill will then make her behave as a 170 BST unit, allowing you to get a higher score ceiling/floor depending on the rest of your team's scoring capabilities.

Giving her R Duel in her A slot will increase her unit score by 8 points, so it should increase your overall average score by 2 points.