Interesting that you're opting for including the decimal point. Jp wiki multiplies by 10 to not have to use that.

R grail needs 4 R's to hit skill lvl 4. 1 + 1 + 2 (lv 3 needs 15 points)

There's more nuance than pure points per R efficiency. An individual also needs to take into account how many R grails, SRs, and SR-grails he currently has, his storage space, and what will be expected to come in soon.
I tend to be more pressured when it comes to space management (I have a ton of spare SRs that I never got around to burning, as I drag my feet on the gem gacha drawing process). I prefer to get my R grails up to lv 3 (thus, I treat them as converting 2 R's into 60 points). The extra 2 R's to push them to lv 4/80 points just isn't worth it to me when another 40+ R grails come in next union event. SRs I bounce between either lv 3 or lv 4, depending on how many points I need exactly and if I'm in the mood for being efficient with the SRs. SR grails might as well be taken to lv 5 though. The 160 points from a lv 4 can be replicated through lv 3 R grail and lv 3 SR. Or a lv 4 SR + 2 Rs if you're running out of the R grails.

You'll see skill levels higher than 20 in the future. Final Limit Break raises the cap to 30.