When exerting, when does opponent modify it?

[ultra]

Let's say I'm playing Coltec and I have Master Endurance in play.
I want to exert to make an attack I play a power blow.

Immortal Title Set Rarity
Generic Master's Endurance (HS1-050) Season 1 Rare
Type Grid Restrictions Gems
Situation 2 [Master][Strength][Toughness]
Text
You may increase or decrease the size of your Hard Exertions by two

And my opponent is regular Connor with 3 allies in play.

Do I have to announce where I'm making the exertion from (hand or endurance) before Connor modifies it?

[prowler7]

Yes, you have to announce where and how many cards PRIOR to your opponent modifying it.