Hero movement usually reveals the monster(s) location, so how would they get first strike? It admittedly happens on occasion but for the most part, player 1 kicks open a door and rushes in and attacks whoever they can reach. Then the other 3 jump in, then the monster's turn. I Homebrew that ranged mobs attack immediately when revealed, but otherwise... I understand you're going for max difficulty but it definitely seems odd to make an edge case (player 4 opening a door with the last of their movement) the basis of the difficulty system.
To step back for a second, I want to thank you again for your hard work on this. It's an amazing tool and my focus on the formulas is intended to be constructive feedback, but probably comes across like criticism. I'll stop beating the dead horse and go back to working on my own stuff now. Thank you again for the tool and for the clarifications. Carry on good sir!