I don't think it matters much how you spread the fork blades. You have to measure the results. Thinking that the blades will go into plastic yield at the same time is dreaming. And you will have to have a means of correcting one blade at a time.

Using the pipe method is pretty simple. Tug one side until it is half the increase further, turn the fork around and do the other side. Results will be very close to symmetrical. Your method may work if the original builder was completely uniform in the heating of each blade to braze and working with identically drawn tubes. And likewise, completely uniform in coldset required after brazing. That's a lot to ask.

Ben