Does anyone have experience of weaning from the breast onto formula, late in baby's first year?

DS will be 11 or 12 months old when I return to work (I may be able to use vacation days to extend my leave by 4 weeks but I haven't decided yet) and I want to wean him in time for this.

I'm aiming to start the process when he's around 10 months old. Is that early enough to do it comfortably (and kindly)? Do I just gradually replace breastfeeds with formula? How on earth do I know how much to give him?? I foresee a lot of wasted formula & money!

Thanks for your help!