Determining New Zombie CR's for 5e

[Tevin]

Hey everyone, I'm currently running a zombie apocalypse type game using these rules for D20 modern conversion to the 5e rule set. I'm making a slew of new types of "infected" since fighting the run of the mill zombie over and over gets boring fast. However, I'm new to content building in 5e and am having trouble figuring out exactly how powerful these creatures might be in terms of CR. From what I can see there aren't any official hard and fast rules for it, so I figured I'd ask for advice here.

Anyone have a link to a good place to steer my on the right path to determining CR? Also, what would you consider the following examples CR to be? I'm thinking CR 1 as it's a simple zombie chassis but with an ability that can increase its damage output. Let me know if you agree or not and why.

Screamer
Medium Undead, neutral evil

Armor Class 8
Hit Points 22 (3d8+9)
Speed 30ft

Str 13 (+1) Dex 6 (-2) Con 16 (+3) Int 3 (-4) Wis 6 (-2) Cha 5 (-3)

Wisdom Save +0
Damage Immunities Poison
Condition Immunities Poisoned
Senses Darkvision 60ft, Passive Perception 8
Languages Understands what it spoke in life but cannot speak

Undead Fortitude If damage reduces the screamer zombie to 0 hit points, it must make a Constitution saving throw with a DC of 5 + the damage taken, unless the damage is radiant or from a critical hit. On a success, the zombie drops to 1 hit point instead.

Actions
Slam Melee Weapon Attack: +3 to hit, reach 5 ft., one target.
Hit: 4 (1d6 + 1) bludgeoning damage.

Scream (Recharge 5-6) The screamer lets out a terrible shriek affecting an area within a 20 foot radius from the screamer. Each living creature in the area must make a DC 12 Constitution saving throw, taking 1d6 Thunder damage and suffering the stunned condition for one round on a failed save, or half as much damage and not stunned on a successful one.