The way I explain it to myself is to think in round numbers of 30 days.
Month 1 - 30 days away
Month 2 - 30 days away
Month 3 - 30 days away
Month 4 - home - as I have now used my 90 days in 180 allowance
Month 5 - home
Month 6 - home
180 days is now complete
Month 7 - away because as each day of the month goes by, a day in Month 1 becomes a day over the 180 day rule. It is replaced by each day in Month 7
Month 8 - away because as each day of the month goes by, a day in Month 2 becomes a day over the 180 day rule. It is replaced by each day in Month 8
Month 9 - away because as each day of the month goes by, a day in Month 3 becomes a day over the 180 day rule. It is replaced by each day in Month 9
Month 10 - home
Month 11 - home
Month 12 - home
etc etc
Please bear in mind that not every month is 30 days so this will need to be accounted for.