Maths: money, and using a 100 square to add

We checked our homework from yesterday, about writing amounts of money using a pound sign. The rule is easy. There must be 2 digits after the decimal point. The pound sign comes at the beginning of the numbers.

Afterwards we did a page of work about using a 100 square to add (and to subtract). Look at the board above to see the rules we followed. Can you make up rules, using north, south, east and west, to add and subtract other numbers?