The reason it's confusing is because it depends what visa you entered the UK on as to when you can apply for FLR(M).
To qualify for ILR, you have to spend 60 months in the UK on settlement visas.
If you entered the UK on a spousal visa, you will have the following visas:
- spousal visa valid 33 months
- FLR(M) visa valid 30 months
So you have 63 months in total, giving 3 months leeway to enter the UK and start your first 30 months... which means you can apply for FLR(M) up to 28 days before you reach 30 months in the UK, which may be more than 28 days before your visa expires.
But if you entered the UK on a different visa (fiance, student or work visa) and then you got married and switched to FLR(M), you will have the following visas:
- FLR(M) visa valid 30 months
- FLR(M) visa valid 30 months
So, you won't reach 30 months until your first FLR(M) visa expires, meaning you can't apply for the second FLR(M) until 28 days before 30 months, which will also be 28 days before it expires.