WowAce

I would like if there was a option to add the cumulative drop rate to the tooltip.

I don't know much about coding addons and LUA, but I can provide the mathematical formula 

That would be accompliced by the following formula:

[Chance of having at least 1 drop at this point] =  1 - (1 - [droprate in percent (e.g. 0.01 for 1/100-rate)]) ^ [attempts]

I really much like the idea, I've been doing this on the side in spreadsheets for what I was farming so I definitely would turn this feature on.

I'm not a maintainer of Rarity in any way but I've been tinkering with my local copy to give an example of how it could look like:

I just added the following snippet line 2446 in GUI.lua

if item.chance then
        local probability = 1 / item.chance
	local luck = 1 - ((1 - probability)^attempts)
	local s_luck = format("%s: |cFF00FFFF%.3g%%|r of players would have dropped it by now. (|cFF00FFFF%.3g%%|r droprate) ", itemName or item.name, luck * 100, probability * 100)
	self:Pour(s_luck, nil, nil, nil, nil, nil, nil, nil, nil, itemTexture)
end

 Of course you would need a bit more to add it as an option in the GUI and stuff =)

Gotcha, created this issue: https://github.com/SacredDuckwhale/Rarity/issues/191

I probably won't have time to implement optional features such as this anytime soon, but contributions are always welcome :)