He's partially right. That dizzy is designed to open up an additional 25 degrees over the rpm range of 1000 thru 4000. Initially you might think that it merely adds 25 to whatever you set at idle, and you'd be right if you set your idle at 1000 rpm, but if you set your idle advance at 1200 rpm rather than the 1000 the dizzy assumes, the dizzy will have already started to open up during that (admittedly small) 200 rpm difference. Maybe that's only a couple of degrees though....
So, if you set 12 degrees at 1200, the dizzy is going to add whatever it has left to offer between 1200 and 4000, say 23 degrees or so, so in the end your advance at 4000 would be 12+23 = 35; just about right in my opinion.