I have been keeping track as I boost monsters. I have worked out the slope and intercept that contribute to HP/ATK/Rest for a number of Monsters. I have posted the equations for others to use. I would love any feedback that the commmunity has for me on these.
Here are the monsters that I have added equations for so far:
The other benefit of these (besides using them to anticipate the HP/ATK/Rest of a companion monster when you only know their level) is to predict the trends of each stat. If a monster has a large slope for a stat that means that stat will increase faster per level than another monster with a low slope.
The slope is the number that gets multiplied by the level.
Here is a graph showing the trend lines for Ashadillo. You can see that he has very low ATK and Rest stats.