1000iu daily for each 25lbs weight + 100iu for each extra 2.5lbs.

It doesn't have to be worked out to the last iu for each day.
D3 is inert, it's only converted to the active forms calcidiol calcitriol as required and this takes time.

So you can multiply the daily requirement by 7 and provide that once a week by using the contents of a dry powder capsule (the covers slide apart the fine white powder can be sprinkled in food or mixed into a liquid fish oil)

25(OH)D postal test will check you are providing sufficient to keep status above 55ng/ml 135nmol/l.

This calculator works out sun exposure for 1000iu but remember either the amount of skin exposed to the length of exposure will need adjusting to meet daily requirement in full.