hey hun

Personally I would state you are taking 12 months leave from this date to this date.
with my work i Have to give one month's notice before i want to return and that's it and I can return at any time in that 12 months. I'd say that is pretty standard so i'd be opting for the full entitlement and then fiddling down the track.

And yes, you can say in your 'as per convseration with boss, I would like to be considered for part time employment upon my return'.

good luck! and before anything becomes official they need to reply with the formal paper work, so don't feel that once you've written this letter its locked in and unmoveable. You present the letter, then they present you a 'contract' sort of letter that you need to sign and which they have signed so you are both agreeign to the conditions. Your first letter is presenting them with your intentions and wants.