When is the count of 180 days start for multiple entries ?

[ros6372]

Hi Gurus,
I’m a Malaysian married to British citizen about 1.5 years ago. I can’t apply for spouse visa till early August this year when my husband meets the financial requirements for sponsor.
I have been in and out of UK multiple times since Dec 2014 as a tourist visiting family. Each entry I got stamp “leave to enter for 6 months…” Question is how do I calculate the number of days I can stay in UK without exceeding the 6months/180 days rule out of 12 months/365 days?
Please find below my visit history:
1. 20/12/2014 till 29/12/2014 (10 days)
2. 02/05/2015 till 09/05/2015 (8 days)
3. 18/09/2015 till 05/12/2015 (78 days)
4. 22/12/2015 till 08/03/2015 (77 days)
5. Now, I’m returning again in UK on 19/05/2016 and planning to stay for 73 days i.e. till 31/07/2016
I have done a lot of reading online and the available information is very confusing. Even in UK immigration site, the way it is defined is very vague.
Scenario 1: Is the calculation from the first entry stamp?
- (365 days) 20/12/2014 till 20/12/2015 : I have stayed for (10+8+78) 96 days
- (365 days) 22/12/2015 till 21/12/2016: I will be staying for (77+73) 150 days

Scenario 2: Is the calculation from the date of last entry and count backwards?
- (365 days) 19/05/2016 till 20/05/2015: I have stayed for (77+78 days) 155 days

Scenario 3: Is the calculation from the intended departure date and count backwards?
- (365 days) 31/07/2016 till 01/08/2015: I will be staying for (73+77+78) 228 days

If correct calculation is Scenario 3, I will be overstaying.
Please help, I do not want to have my spouse visa application rejected for over staying.

Thanks.

[vinny]

Strictly speaking, on each entry, you may stay up to 180 days or until your leave expires, whichever is sooner. However, they may consider your past immigration history to determine if you are a Genuine visitor under V4.2.