Epic vs Rare power difference

Hey all! I'm just curious to know the power difference between Epic and Rare weapons. For example, what would be the Rare equivalent to a maxed out Epic Missile Launcher Tier II (level 20)? Would it be a maxed out Rare tier III (level 30) or IV (40)? Maybe something in between? The information would be a huge help in managing my upgrade decisions, so any feedback would be very much appreciated. Thank you!

 

https://docs.google.com/spreadsheets/d/1YBXS0LPaEQwDUyXe4xJ-05DQV72_m68JZNKAkUOOVdg/copy?usp=sharing

That's a spreadsheet for item power comparison. According to it, you would need a lvl 31 rare to match the power of a lvl 20 epic.

 

Or you could just use simple math as I like to do and go like T1 epic = T2 rare = T3 uncommon and be close enough for a ballpark mortar...