I'd like to give a suggestion, but also to others to check if this works:
I was using the predictor to catch the Prehistoric Floors for farming Pepper Rex for the monster slaying hero check. Now, i was reading that it was not 100% guaranteed, which i confirmed, but i did notice something i tried. When you start the game, it seems the levels it states in the predictor are correct, but as you go the days continuosly, it gets more inaccurate. But, out of curiosity, i tried something at the end of the day, after you go to sleep and save the game, instead i exit to the desktop, and restarted the game. The level it stated was the correct one, and i could get to the Prehistoric Floor. Even more, it can be repeatable, meaning, you can leave the mines, and go back to the same level at that day, and the prehistoric floor is still there.
I didnt had time for a proper test to see if this can be repetable, but if others can try it, and if it is confirmed, suggest this workaround for those looking for Pepper Rexes.