When creating the monster, do you apply resistance and immunity multiplirs separatly?

[stenver]

Let us build an easy custom zombie

Atck +3 dmg 1d6+1 => Attack CR 1/4

Hp 21, damage immunities: Posion, Cold, Piercing. Damage resistances: Bludgeoing, magic weapons, fire

When calculating effective HP, is it
21 * 2(resistances) * 2(immunities) = 84
or just
21 * 2(resistances,immunities) = 42

Another question,

when calculating average CR and you have a tie, is it simply your choice wether to round up or down?
e.g.
Defence CR 1
Attack CR 2
Decide wether CR 1 or 2 on your gut feeling

Thank you

[Chaosvii7]

Worth mentioning that you only apply resistances and immunities that hinder the party, like a magic weapon resistance against a party that has no such equipment. Fire and Cold might be roadblocks, but if your party doesn't deal excessive amounts of Poison damage then it shouldn't be a factor to weigh in the design of the monster.

Also, if your CR isn't a whole number, round up instead of down, contrary to what you normally do when encountering decimals in D&D. It's safest to assure that your monster is faced at the most appropriate level to its best capabilities.

Assuming that the party has neither magic weapons nor poison damage, but spellcasters do primarily fire and cold damage, then its effective HP would be 84. AB but I'm pretty sure the math that got you there is right. As long as you ignore superfluous resistances and immunities and only factor the ones that get in the way of the party's own DPS capabilities then you should be fine.