Individual rather than game-wide use is the intention of the Variant: Spell Points option. It's in the "Dungeon Master's Workshop" chapter, which has lots of advice and ideas for modifying the game to suit your table. Some of these are spelled out as game-wide. For example, under Combat Options, there are "Action Options", and the book says:
This section provides new action options for combat. They can be added as a group or individually to your game.
(That is, a group of all of these options, or just individual added actions. But, clearly, game-wide.)
On the other hand, this variant is under Creating New Character Options, and specifically "Modifying a Class". It says:
The classes in the Player’s Handbook capture a wide range of character archetypes, but your campaign world might have need of something more. The following section discusses ways to modify existing classes to better serve your game’s needs.
I think it's pretty clear that the intent here is for this option to be an example of a way to build a new, additional option to broaden the range of archetypes. Other sections here make it clear that this wouldn't need to even apply to all examples of a particular existing class. For example:
For example, you could decide that the clerics of a particular deity belong to an order that forbids the accumulation of material goods, other than magic items useful for their divine mission. Such clerics carry a staff, but they are forbidden from wearing armor or using weapons other than that staff.
So, come up with some reason why these druids are different. I might even consider going as far as to create a new druid circle that accesses magic in this way.
N.B. I have no experience with this option in play, and don't have particular knowledge or even opinion on whether it is unbalanced per se. I know answers which have that are preferred, but in this case, I think it's objectively clear that the option is meant to be a mix-in, not a game-wide replacement.
This is a case where specific beats general. Book of Ancient Secrets does exactly what it says, which is that it allows you to:
[A]dd other ritual Spells to your Book of Shadows. When you find such a spell, you can add it to the book if the spell’s level is equal to or less than half your warlock level (rounded up)
3/2 = 1.5 ~= 2. You can transcribe any and all rituals your warlock finds that are <= spell level 2. Detect Magic is level 1 and a ritual. 1 < 2, so you can transcribe it.
Best Answer
There's no point to doing this. The idea behind spell points is to have a single resource fueling your spells regardless of their level. However, Warlocks already have that, because their spell slots are only of a single level.
With that said, if you insist on doing this, the correct progression would be:
This is a direct translation of the Warlock's spell slots at each level into spell points; which is exactly how spell points are calculated for all other classes.
Note that if you use this system and allow the Warlock to create lower level spell slots than their maximum, you are fundamentally changing the Warlock class. I would carefully consider balance before I did this. Of course, if you don't allow them to create lower level spell slots, there is absolutely no point using the spell point system, since it will just be a direct translation of their spell slots.