Turns out a reroll is equivalent to adding five to a roll. Sometimes let’s just exclude the randomness.
Focus on the goal! When you use a hero point, you can elect to either reroll the die or add +5 to the dice value. If the new value would instead cause a critical either as a natural 20 result or the total result being 10 over the DC, treat it as if it was a critical . Dice values over 20 do not cause a critical result unless the total is 10 over the DC as normal.