I thought rim tape was supposed to be 5mm wider than the internal width.
That’s one of the traditional ways, but as rims have got wider it’s become practical to tape the holes not the rims- it’s more durable ime, since there’s no risk of the tape getting damaged as the tyre comes off the bead. But it won’t work on everything- some rims are too narrow, some are badly sealed and need the tape to be airtight.
Worst case scenario is for the tape to be just narrower than the rim (one taking into account the amount that’s used up in the well- if the tyre sits off the tape but it goes up on the bead, or the tyre’s half-on-half-off, then it’s far more likely to push the tape off when you change tyres.