Step #1: 
To calculate the correct reamer diameter, based on the machine ability of the material to be reamed, deduct the allowances in the following table from the maximum finished hole size:

For example: If your finished hole size is .5000 with a -.0005 +.0000 tolerance, and the material is 302 Stainless (which has a machine ability rating of 50) the chart shows that the range to deduct is. 0004-.0005.

.5000 -.0004 = .4996
.5000 -.0005 = .4995
Therefore, the best reamer size is .4995
 

Step #2: 
Based on the reamer selected in step # 1, the following table shows the amount of stock that should be removed by the reamer (leaving the recommended amount will ensure the best finish and tolerance):

Using the same example (finished hole size is .5000 with a -.0005 +.0000 tolerance, and the material is 302 Stainless (which has a machine ability rating of 50): the chart shows that the amount to deduct from the reamer size is .007-.015.

4995 -.015 = .4845 
4995 -.007 = .4925 
Therefore, the best hole size before reaming is between .4845 and .4925 
 

Step #3: 
Based on the pre-reaming hole diameter selected in step # 2, the following table shows the oversize allowance that should be deducted from the hole size to arrive at the correct drill size:

Using the same example (pre-reaming hole size is between .4845 and .4925 and the material is 302 Stainless (which has a machine ability rating of 50): the chart shows that the amount to deduct from the Yankee reamer size is .0048. 

4845 -.0048 = .4797 
4925 -.0048 = .4877
Therefore, the best practical drill size is 31/64 (.4844) because it is between .4797 and .4877 To recap: in this example you should use a 31/64 drill and a .4995  reamer.