Well, you need to floor the fractions to integers, so 2.6=2, BUT in general, yes, you can expect about -15 armor when all pellets hit; even more since extra armor loss might be caused by penetrations.
Oh yes and the armor damage is directly dependant on the damage roll, so each pellet can put 0-5 damage on armor (as max. damage possible is 200%*26=52).
Also in-game text:
STR_PINUP_023_UFOPEDIA: "Chem can melt your opponent right out of their armor. All chem weapons cause armor damage equal to 0-20% of their Power, before armor's defense is even considered, unless otherwise stated in the Pedia. This makes subsequent shots more effective. The effect stacks with armor damage caused by any penetrating hits, if applicable. Chem also causes moderate Morale damage."