Larian Studios Forums Baldur's Gate III Baldur's Gate III - Build Discussion Loss to ESL from Fighter vs. Ranger

I find ESL calculations confusing, so I'll ask - what is the difference for a Cleric in taking a lvl of half-caster versus zero caster?
A 6th lvl Character, 5 levels of full caster and 1 Fighter, has ESL of 5, that I understand. What if 5 levels of full caster and 1 Ranger? is the 5,5 rounded up or down?
I suspect that this is childishly simple, but I can't wrap my mind around it ...