My reasoning: experience and mentally recorded number of elixirs it takes per level, for me, on average.
I level normally with a rogue two rangers and myself (a druid). We level in the right place and level at the same speed as each other so the XP gain is pretty much the same as solo (AVG = 1.1k XP per 1* kill). According the the graph (60s per 1* kill) I would need 8 elixirs to level - however with this group we average 30s per 1* kill, meaning that it should take me roughly 4 elixirs to level - which is exactly what it took me to get to 114 from 113. No doubt it will be the same 114-115.
It works!
Unless I am a bad judge of timing and our real AVG kill time per 1* is actually 15s
