How do I tell a field which references another field that the original field has changed and then to reflect those changes ?

ie.

Field A is a sub-total value

Field B points to Field A to show the same value but further down in my form.

When Field A is updated , how do I tell Field B to reflect those changes ?

Or 

If Field C uses Field A's value in a calculation.

When I change Field A, Field C's value doesn't update based on the change done to Field A.

ie Field C = Field A + 99

If Field A = 1, Field C = 100

If Field A becomes = 2, my Field C still shows 100, not 101.