Potion Event

I want to get the level of a potion, the potion and the item in the brewing stand. I saw the BrewEvent, but the event is called when the potion is complete! What I can do to detect that? For example: If the Brewing Stand has 1 potion of Speed 2, allow to create a custom potion with Speed 3 with glowstone.
I should create an event (I have no experience doing that), or use InventoryClickEvent? And how I should use it?

 

What i might do is add a custom recipe that gives them the speed 3 potion so you might put the brewed speed 2 potion and then surround it with four glowstone dust then you get the custom speed 3 pot

 

@CoderRex That method is easier, but I want to try to do custom potions for Brewing Stand.

 

PotionBrewer
PotionEffect
PotionEffectType
PotionEffectTypeWrapper
PotionMeta
PotionSplashEvent
PotionType

This is what i found on potion when i was on bukkit page thingy check this