Developing computaion skills and learning number facts is a hot issue everywhere. I've found some success, both in terms of student achievement and administrative and parental support, for giving daily homework practice and weekly quizes using things like adding "combinations of 100". Here are some examples:
40 + 55 + 75 + 20 + 45 + 40 =
80 + 25 + 230 + 85 + 110 + 25 + 50 + 10 =
I give a few problems eaach night, with a small space to show how the kids "chunked" the numbers, then give a weekly quiz using the same or similar problems. This grew into the "mental math challenge" which was a class tournament. The kids loved this, and the parents were amazed that the kids could figure out the answers faster that they could using a calculator.
This type of calculation practice seems to me to be supportive of the philosophy of Investigations. I'd love to hear what other teachers have come up with. I hope this helps.