But remember that this leaves you with no holidays or any slack anywhere... If you want to have a couple of weeks off in the summer, for instance, and perhaps the odd day else where you would be better doing 21 hours and keeping tabs on it all so that you could build up a day's leave in just over 2 weeks.
I find working it out in annual hours is the easiest, so a full timer would work 52 x 35 hours a year (1820) MINUS 8 bank holidays (8 x 7 = 42) MINUS annual leave (25 x 7 = 175), so a total of 1603 hours.
If you're 50% of FT you'd work 801.5 hours a year.
Working 1 full day a week during hols is 12 x 7 = 84 hours, leaving you 717.5 hours to work during term time, so 717.5 / 40 = 18 hours a week.
Double check what full time is as it seems a bit low but basically your maths looks good to me. It works out to 2.9 days per week term time so what you might do is convert everything to hours. i.e. 17.5 x 52 = hours per year. Knock off the holiday time hours you want to do and then agree to work the rest at a rate of approx 3 days a week term time, with the odd slightly early finish. What you're asking for is annualised hours.
Unless you work the same number of hours everyday, then again it's best to work out your holidays in hours. You should also get an allowance for bank holidays. Happy to calculate that for you, too complicated to type on my phone!
I've just been offered a new job (yay!) that's 17.5hrs per week - because it's working with schools, I've asked if I could up-weight my hours during term time and do less during school hols. Obviously this would benefit the organisation I'd be working for as I'd be more productive when the schools were actually available. They seemed keen on the idea but I'd like to go with a proposal to them about how this would work.
This is the info:
17.5 hrs a week (they are flexible about how these are managed) and 25 days annual leave pro rata (assuming that would be 12.5 days).
I'd like to suggest that I work one day a week (7hrs) during the school hols (12 weeks a year).
How do I work out how many hours this would leave me to work during term time (40wks) - I'm assuming I wouldn't need to factor in annual leave into this?