In my understanding, I must be physically present in the U.S. for 30
months in total during 5 continuous years without leaving the U.S. for
more than 180 days at a time before applying for citizenship. Does
that mean that I need to be in the U.S. for more than 183 days every
year? Or, is it OK for me to be in the U.S. for, say, 170 days in one
year and then 200 days in another year to make up the missing days, to
reach 30 months in total in 5 years?

Hi nanagomamie,

There are two residency tests that you must pass, neither are
interpreted yearly but rather are based on a term of 5 years.

1)None of your absences may exceed 6 months (Continuous Residence Test).
"Continuous residence" involves the number of days you were outside
the United States during a single trip.

2)  You must be in the U.S. for at least 30 months (Physical Presence
Test) within the 5 years.
"Physical presence" involves the total number of days you were outside
the United States on all of your trips.

A GUIDE TO NATURALIZATION
http://uscis.gov/graphics/services/natz/English.pdf

So, if none of your trips last for more than 6 months, you may make
any combination of trips that you want within the 5 years (lots of
short trips, or fewer long trips), as long as you are physically
present in the U.S. for at least 30 months in the 5 years. Also, as a
side note, don't move within 3 months of applying for naturalization
and you may file for naturalization up to 90 days before you meet the
continuous residence requirement.

What is the purpose of this worksheet?
"The Eligibility Worksheet will help you decide if you are eligible to
apply for naturalization."
http://uscis.gov/graphics/services/natz/wsinstruct.htm

I hope that's clear. If you have any questions, please post a
clarification request and wait for me to respond before closing/rating
my answer.

Thank you,
hummer