PL is a hodge podge. They seem to have different drop rates in each campaign. But in DL they tell us the rates.

Yes, I think 11.5 boss/ghost/executions for the higher percentage (shorter duration) elixir is about right. I said 1 in 15 with fuzzy math but it came to 7.98% with real math (which you're right, becomes 9.08% when you add in the normal chance for a pink without the luck proc).