I think I figured out how Civ 6 Religious Pressure works.

So I was working on adding a religious pressure tooltip for my Simple UI Adjustments mod, and I may have figured out (to some extent) how Religious Pressure works.

So the way pressure works is ALL the religious pressure a city has ever been subjected to is added to a pool. Citizens are then divvied up according to the share of religious pressure a religion has in the pool. Since religions can't have half a citizen, the amount of citizens is always rounded up or down to the nearest whole number. I.E

Amount of Followers = Round(CityPop * (Religion's Pressure/Total Pressure))

To illustrate, the tooltip from my mod: http://i.imgur.com/Ep5ZlbF.png (note this is DIFFERENT from the data on the Religion Lens)

Pantheon has 2 Followers:

3 * 150/280 = 1.6 (Rounded up to 2)

Orthodoxy has 1 Follower:

3 * 94/280 = ~1

Catholicism has 0 Followers:

3 * 36/280 = 0.38 (Rounded down to 0)

So those numbers that appear when you spread religion or kill an enemy in theological combat are the exact change in pressure that occurs in cities. Like Civ 5, cities with a dominant religion will also spread their religion passively, though I'm unsure of how exactly the amount of pressure it exerts is calculated. That said, the numbers are pitiful, especially if you're dealing with a city with an entrenched religion.

I suspect Holy Cities push +4, Cities with a Holy Site push +2 and all other cities push +1, but I'm not certain.