RTF is a global modification that replaces all towns in the game.

If you work on the principle of "created a monster - calculated its stats," then everything is fine.

I disagree. A calculator is an incredibly useful tool that simplifies the process of balancing creatures; however, you should consider factors that the calculator cannot calculate:
1) Ease of defeating neutral creatures.
2) The combination of creature abilities and stats with the racial ability.

And so on.