Simple duality question

Happy Sunday Night Everyone

So, I've read the relevant sections of the BIM in regards to duality along with reading other publications.

On the one hand I am told that where an expenditure has a dual purpose of a business and personal use the whole expense is disallowed.

Then I read that where you can apportion the percentage between business and private the business part is allowable.

Hmm

1. Is it the case that the apportionment is only applicable to a limited amount of scenarios or is it simply the case that where duality exists that's the end of the matter in all scenarios?

2. Assuming apportionment is allowable, how exactly do you justify the percentage beyond sticking your finger in your mouth and then holding it to the wind? Obviously some thought has to go into it but from what I have read it's not exactly an exact science.

Many thanks in advanced for any response. I just love learning more about tax and this forum is very helpful.