How much does premade size add to key fragment drop rate?

It's very difficult to figure this out due to the sliding drop rates over the course of a month, lack of consistent premade sizes, small sample size with an apparent limit of key fragments we can find in a month, and my lack of desire to play solo as all I know is that it would reduce drop chances, so can a Rioter just give us an idea of how much benefit we gain from going through the effort to get a 5 man premade together?

There are a number of ways it COULD be working that all influence how much effort I should put into finding that fifth man before going into the queue...

Additive: it gives a simple flat boost to drop rate per member of your premade, say a 3% boost per member, so a 5 man premade may increase the droprate from a base of 10% to 22% (4 additional members * 3% + 10% base). I highly doubt it works like this as it would make it more influential later on in the month while barely being visible early on, and it seems like almost any nonzero value would end up with a higher droprate than I'm experiencing in my full 5 man premade games.

% boost: This appears to be the most likely, something like a 20% boost per additional member in your premade for a total of a 80% boost in a full party (aka 1.8x droprates, NOT a key fragment dropping in 80% of games). This would scale appropriately with reducing droprates and it doesn't seem too far from my experience in key fragments dropping from wins.

Multiplicative: Functionally the same as the previous at a value of +100%. This seems unlikely given I had such a low drop rate in my premades and solo games would be only 1/5 of THAT drop rate yet people, myself included seem to be reporting more key fragments from solo queue wins than would be expected if they were so rare. I include this method because it is basically the same effect that premades have on getting chest drops (5x the number of players who can get an S to qualify you for a chest on whatever champion you're playing) and it seems odd that the only method we have to increase key drop rates seems much less effective for keys than it is for chests which everyone thought was going to be more about champion mastery than simply getting a premade.

Stronger scaling: Some stronger form of scaling (quadratic?) based on party size. This also seems unlikely since the increase would be even more drastic than multiplicative. This possibility is only included because of the roulette system riot used for party rewards a while back where 2 man premades had almost no benefit while 5 man premades had increased odds of a bonus AND access to higher tier prizes.

Key fragment drop rates seem frustratingly low with little you can do to affect them while chest drops are hilariously easy to get simply by doing the exact same thing that is supposed to help you get keys. Riot said that some people were supposed to get more keys and others would get more chests but it seems pretty clear that opening chests is completely gated by key drops while chest drops are trivial for the vast majority of players.