Why not just look at the money supply to calculate inflation? I see a lot of debate about the right measure to use to determine inflation -- apparently the CPI overstates it, etc. Why not just subtract the GDP growth rate from the money supply growth rate (or to be more accurate, divide 1+R by 1+r)? In the long run, it's supposed to predict inflation perfectly, yes?
To an extent yes, tracking the money supply is somewhat helpful in determining inflation, but there are other important factors to consider.

Productivity?

Say there was a massive crop failure and the same amount of money.  Food prices (and prices in general) would go up, right?  

What is money?

Do we just count government money (monetary base)?  There are aggregates for that, but if you've accepted a check for services before you know that you also accept bank money which does not have a 1 to 1 relationship with government money.  So you have to include bank money to determine how much money is the economy.  But what types of bank money?  Demand deposits?  M1?  Interest bearing deposits M2?  M3?  This is subjective...but it helps to look at many aggregates.

What about foreign trade?

If you count the money supply do you include say dollars in the USA or dollars abroad as well?  In some cases dollars are used a medium of exchange abroad and don't really come home, so they don't influence domestic prices like domestic dollars do.

What about foreign exchange

Currencies compete, and because they are fiat there is always the possibility that one currency (especially a smaller one in a weakened state) will suffer a confidence crisis and crash.  This can influence prices, yet the amount of money in the economy can stay the same.

So truely, you need to look at many factors to figure out inflation.  All that being said though the supply of money is still something very important to keep an eye on.