Oh dear what a discussion this has turned out to be.
Interesting that discussion in Spanish translates as argument.
I agree that it can be confusing.
Yes there is a calculator but if you're a pen and paper person here goes.
Split your paper into 4 sections of 90 days each.
Day 1 stays as day 1 .
You can stay 90 days out of the first 180.
It can be in chunks of your choosing but always calculate from the original day 1.
Then you've got another 90 days.
It can be confusing as it could be interpreted as 90 days here. 90 days somewhere else 90 days here 90 days away.
Really hope this is helpful.
It is explained by David Searle in EuroWeekly 11 Nov.
Please everyone let's stay helpful.
Vitriol is wasted energy which could be better spent enjoying life.