Steel has a property called spring back. It's the same as you flex anything elastic. First you're working within the elastic range where it flexes but will return to it's original shape. Beyond the elastic limit, it'll bend permanently, but the same spring back will remain.

So if you want to spread your fork 4mm, you have to 4mm beyond the elastic limit, then when you let go it's spring back, but end up the same 4mm away from where it started.

It's hard to feel when you're working with a screw that way, so I prefer to use a gauge, and bend by hand where I can feel the point at which it's bending. I know because it requires progressively more force to flex it, then suddenly it moves withour needing more force.

I also prefer to bend each blade independently half the distance, gauging on the other. If you bend both together, the weaker one will bend while the stronger one is still in flex mode, so you end up with one blade doing all the bendiing.