David B wrote:
How can I get the monthly rate from a compounded
annual interest rate?

I assume you mean "the monthly compounded rate from
the annual percentage rate (APR)".

Some people will say: simply divide APR by 12.

But if the compounding frequency is monthly, the
correct monthly rate is =RATE(12,,-1,1.045) for an
APR of 4.5%, for example (1.045 = 1 + APR).

However, even when the rate is specified in monthly
or annual terms, it is often compounded daily. In
that case, the monthly rate will vary depending on
the number of days in the month. A "typical" monthly
rate is =FV(RATE(365,,-1,1.045),30,,-1)-1.