Sure, just output a voltage V = RPM*5.0/100.0
As you can see, V=5 if RPM=100, V=2.5 if RPM=50, etc.
The Adafruit tutorial I linked shows you how to output a voltage.
Keep in mind that 5V output from the MCP4725 corresponds numerically to 4095 at 12 bit resolution, so the formula that you will actually be using is V=4095.*RPM/100.