View attachment 2812466 If you really feel the urge to tweak your string spacing, then you have the wrong bridge. You need one of these...

However, with that Fender bridge? As others have said - put the strings on the grooves that make
you happy with
your string spacing. Then measure it carefully; write it down; and move on to your next obsession. There is no universally optimal, carved-in-stone spacing that makes us all universally happy; sorry...However; to answer your original question: how do
I adjust the string spacing on a vintage Fender bridge (or any other bridge, for that matter)? I don't, really. I get some tension on the strings, so that they're straight between the nut slot and the anchor point in the bridge/tailpiece; and let the string spacing more or less adjust itself. On most of my 22 basses, string spacing adjustment isn't really an option, anyway. It just is what it is. Don't let me stand in the way of your obsession; I understand perfectly where you're coming from. And most of us here do, too. We just don't want to see a fellow bass player get so carried away that he loses track of the object of the exercise - which is, to
play the thing...