You must have at least 25 points for an add on, but it can be ANY number above that. There is no real incriment if you are adding on at least 25. In other words, you could add on 26, 27, 28, 999 or more!...you name it. I believe it must be at least 50 for financing through Disney though.
thanks I wasn't sure if you could only added on in certan amounts and our guide was off the phone before i thought to ask. will sit with the calculator now and try not to get carried away