That calculator at http://www.nitrorc.com/fuelws/oilonly.asp is wrong. No matter how much oil you add to a given fuel the relative proportions of the actual fuel (the methanol and nitro ratios) remain the same. That calculator is treating oil as a fuel, which obviously it isn't.
The calculator is correct; You are incorrect
When you add oil to a previous blend with no other changes you no longer have a gallon of fuel. You now have a gallon 128oz + the amount you added so...the percentage of oil goes up and the percentage of every other ingredient goes down.
Example; take one gallon (128oz) of 10 % nitro 17% oil fuel and lets say you want 22% oi
The current fuel has, 10% nitro content and 17% oil content, leaving 73% methanol.
In order to achieve 22% oil you will simply need to add 7.8 ounces of castor/synthetic oil to your 128 ounces of fuel.
When you're done, you will end up with 135.8 ounces (1.06 gallons) of newly blended, custom fuel.
Your Custom Blend Results will be:
Starting Nitro: 12.8 oz =10%....Ending Nitro: still 12.8 oz = 9.4%
Starting Oil: 17% =21.76 oz Ending Oil:29.56 oz = 22%
Starting Methanol 73% =93.44 oz Ending Methanol still 93.44 oz = 68.6%
The combination of ALL the ingredients is called FUEL and the percentages refer to the new total quantity 135.8 oz