The is no diference difference between those server since minimum respawn time is at 500 players and maximum at 800 so it will only vary between those numbers so calmera and mitigera Calmera and Mitigera should have the same respawning time there was a similar question that was answered with the information you were looking for
https://www.tibiaqa.com/18074/there-is-a-myth-that-respawn-depends-on-people-online-is-this-true?show=18074#q18074
About the formula there isnt isn't any information to the players on how much its varies and i dont I don't think anyone have made any tests on how fast a monster takes to spawn at diferent different players online