nanny share costs

[amarmai]

Nanny is shared with a neighbour who is pregnant with 2nd child. We are not going to have a 2nd . At present cost is shared 1/2 and 1/2. The 2 current children are full time at school. How should the cost be shared after the new baby becomes the 3rd child in the arrangement? 2/3 to 1/3 does not seem fair considering that most of the nanny's time and energy will be for the new baby. Does 3/4 to 1/4 seem fairer?

I would have thought the fairest way is to calculate the hours she will have solely baby as 100% other family cost then 50% for the hours school children are home for term time then 1/3 - 2/3 during the holidays and calculate average hourly rate. I'm assuming care will be in other families home (if not now then when baby arrives) so you won't benefit from nursery duties.
If cost is a factor, I'm wondering whether you might find a childminder more suitable going forward?

thanks phoebe. Your % make sense.