Protect the Planet halves the rest.  So you have +11% + 16% + 10% - 20% = 17%

cut in half from your policy is 8.5%

Thank you to both AdamMG and aerez4546 for the explanation. It is interesting to see how some percentage values are additive and some are multiplicative.  Hopefully, some of the programmers are reading these threads and agreeing that the numbers need to be presented in a better way. My first suggestion would be to show the math in the popup that appears when you hover over the pollution number. My second suggestion would be to show the additive numbers as decimal numbers (i.e. .5, .1, etc.) and the multiplicative numbers as percentages.

This also raises the question of why Pollution is expressed as a percent.  What is it a percent of?  What happens at 100%?  Would it be better to just show the level of pollution as a decimal number?  That way you could have the additive numbers as decimal numbers and the multiplicative numbers as percentages.

Hopefully, these musing will inspire the programmers to improve how they communicate what pollution is and how (mathematically) the level of pollution is affected by what you choose to do.

"From what I understand, 1% of pollution is exactly enough to reduce food production by 1%.  There doesn't seem to be any "hard basis" for the numbers, other than that."

AdamMG,

It looks like you are correct.  The below screen snapshot is from one of my worlds where the pollution level is 14% and the popup window for my food production does show a reduction of 14% for pollution.

It is interesting that you bring-up growth because that is another puzzling value.  Growth Per Month is expressed as a decimal value, not a percent, but all of the factors (except for the races) are percentages.  If I sum the 2 race values (9 + 3) I get 12.  If I sum all of the percentages, I get 458.6%.  If I multiply the sum of the values by the sum of the percentages, I get 55.032, not the 67.03 that is shown in your screen snapshot.  Therefore, there must be some unknown method of doing the calculations similar to the pollution calculations that should be explained better.

Also, while I am on the topic of growth, what does the Growth Per Month value mean?  Clearly, you are not getting 67 new citizens every month so the 67 must be some fraction of a citizen.  What does it take to get a new citizen?  100?  It seems like the amount of growth required to get a new citizen increases with each citizen you get.  How do we determine the population growth curve?  That would help in deciding what options we want to take with respect to increasing a planet's population cap.  Why would we want to increase a planet's population cap beyond the point where the growth curve goes asymptotic?

There's an ideology choice you can make that I think reduces pollution globally by 10%. But I agree it would be useful to have core world improvements that would reduce it like we already have for crime.

That is good to hear.  We understand that the game is a work in progress so there are still a lot of rough edges.  Hopefully, you will take the time to look at ALL of the parts that involve math (pollution, growth, crime, etc.) from the viewpoint of the player and make modifications that will make them easier to understand so that they can make informed decisions.