You’re right about the angular momentum. THe planet’s round though. So I’d expect the order of magnitude for that kind of a correction to be smaller by less than 2. Earth’s is ~0.3% at the equator.
I know that earth's effect is pretty small. As I recall though, there's a tendency for large-mass solid objects (especially close in ones) to spin like the Dickens at some stage in their development until they transfer their momentum and become tidally locked, which is part of what has happened to the earth because of the moon. I only mentioned it because in certain conditions it might be sufficient to nudge the figures a bit. OTOH, if this thing is close enough in to have a 13 day "year", on an object as long-lived as a red dwarf, it's probably tidal-locked or nearly so.