Advantage: Serendipity

I've never seen this,
but I've always liked the title.

Serendipity is a pretty difficult advantage for me to wrap my head around. I mean, compare the advantage very similar in concept and theme Luck, and it is a whole different ballpark. Luck has a very hard coded mechanical specific benefit to it, and Serendipity has a really wibbly wobbly soft and mushy benefit. Which kinda stinks, because I really like the concept of Serendipity, explaining the idea of lucky coincidences as a component of what defines a character is pretty swell, and there are a lot of characters in fiction that are repeatedly saved by Deus Ex Machina, so it is not an unusual element either. Today, I'm going to dissect this advantage and try to make sense of it so I can make it consistent.

What Do We Know?

I got this book just because
someone said it might help.

Well, more specifically, what do I know? What I know is that small examples of what constitutes a fair use of Serendipity peppers the books that I have. It is described on p.83 of Basic Set - Characters. Page 73 of GURPS Powers introduces two versions of the Wishing modifier to the advantage. In Dungeon Fantasy 11, the Cleric ability, Great Prayer on page 22 is Serendipity with a Cosmic modifier. Power-Ups 5: Impulse Buys has some options to change some of the mechanics of Serendipity to use the mechanics discussed in its Player Guidance chapter.

As much as I wish there was a way to force a jell-o shaped peg into a square hole, some of these options contradict each other (though are obviously not meant to be used together) and my very cursory research makes it not much more clear than when I started.

What Do We Want?

Again, more specifically, what do I want? I want something that I can manage in a way that I can make sure that my players get their fair share of benefit from the points they spend. Piecing together the rules for each of the books in the way I like it, and combining some house rules

• As in Power-Ups 5, the advantage gives you Serendipity Points.
• I feel like the extra cost of the benefits given by the modifiers for Great Prayer from Dungeon Fantasy 11, and the Wishing modifier from GURPS Powers overlap a bit with my expectations of what Serendipity should be giving for free anyway, certainly Great Prayer's Cosmic enhancement is incompatible with the point system from Power-Ups 5, but Wishing in my head is more like a "Granted only to Self" version, but then, maybe that is what the designers intended anyway?
• Based on the way the advantage is described in Basic Set, there seems to be a rollover effect baked into the advantage. I would say that using the advantage the way it is expanded in Power-Ups 5 means that one can accrue up to two times as many Serendipity Points as their Serendipity levels afford, but only one full batch is added per session. (EG: If someone has enough Serendipity to refresh 4 points a session, a player may store up to 8 over the course of two sessions before extra points are wasted.)
• On that note, a limitation of -1/3 (Not writing it out as a percentage) can be applied to Serendipity to remove the built-in rollover advantage. This specifically ties into my next point because it makes the math nicer.
• Finally, basing it on the "Serendipity Point" mechanic from Power-Ups 5, I would houserule the advantage to be paid for in increments of individual points, EG: using my house rules, one level of Serendipity now costs 7.5 character points (round up,) but only gives 1 Serendipity Point.

Other Thoughts

Any opinions on my thoughts here? I'm really open to suggestions. I specifically bought Impulse Buys on an impulse because someone told me it might help me nail this thing down, but it looks like it comes down to GM judgement in the end. I kinda want to shift the onus to not squander the advantage onto the player, allowing them to give me ideas for me to accept or reject, instead of relying on me to find the correct time to use it, which might not have the cathartic "yes, those were character points well spent!" feeling since you can't tell if that lucky break is me being nice, or because you paid points for it... though that is also a kinda fun feeling I can understand, "heh, the stars lined up just right again for me."

Which makes me think... maybe I should think up a way to execute a usage of this advantage by my volition as well. Maybe I could pass a note to a player saying, "I want to burn your serendipity points for secret reasons, Y/N." But that could ruin suspense too! Maybe I can only have it one way or the other. Maybe I'd make it a small limitation to have Serendipity only run in a "secret" capacity.