Remaking DEF into Dodge/Flee Rate - a simple C++ question

[Slime]

I'm remaking def into sort of dodge system. I want it to work like this: if the damage dealt to the player is lower or equal to his defense rate there's 80% chance he will dodge it and 20% chance that he will get 100% damage (not including ARM reductions). I'm almost there

if(checkDefense && hasDefense)
{
int32_t maxDefense = getDefense(), minDefense = 0;
damage -= random_range(minDefense, maxDefense);
if(damage <= 0)
{
damage = 0;
blockType = BLOCK_DEFENSE;
checkArmor = false;
}
}
The only thing I need is to make "damage-" value being chosen randomly out of these 2 values, not between (20% chance for minDefense aka 0 and 80% chance for maxDefense).

@Edit
Now I think it's not all that has to be done. If random_range chooses maxDefense and damage is higher than maxDefense, the damage will be reduced which is the thing I don't want. When dodge fails there shouldn't be any damage reduction (except for arm). Can someone help me with reworking this?

 

[Slime]

bump
@edit
The way it's now:

if(checkDefense && hasDefense)
{
int32_t maxDefense = getDefense();
int32_t fleefail = random_range(0, 100);
if (damage <= maxDefense)
{
if (fleefail<=80)
damage = 0;
blockType = BLOCK_DEFENSE;
checkArmor = false;
}
}

But it doesn't work for some reason, still if damage>maxdefense player gets always hit and when damage<=maxdefense he always dodges.