Yup your method is just as good, the only draw back is you have to remember whether to add or subtract, and so if you don't use this process on a regular bases, you could get confused and subtract intead of addition, mine uses Subtraction for both scrambling and descrambling. I would say a wee bit better (though i don't like self praising!)
The key difference in your method and mine is that You subtarct your special memorable number from the pin, and then come up with the scrambled number, and to recover your pin number you are adding your special number to your scrambled number,
whereas, I simply Subtract my pin number from the special memorable number and arrive at my scrambled number and if I now forget my pin, I can simply Subtract again the scrambled number from my special number.
In my method you just got to remember that you only need to perform Subtraction., so no need to worry about whether to subtract first or add first as in your method. mine is (not self praising) a little more tweaked concept!
however, both methods are good, and anyone wanting to scramble passwords or pin numbers can choose whichever method they prefer.
The whole idea is to never write down your Pin numbers as they are without first using some scrambling technique.