Here's an old trick from my radio controlled modelling days...
Take a long string and put a piece of masking tape on it. Fold it over the string like a tab as if you were trying to make a little flag. The tape will stick to itself very well, but if you work it once or twice, it'll stop sticking to the string. It'll still be stiff when you slide it up and down, though, so it won't move around when you measure the other side.
Make a little arrow on the masking tape flag. Now, you have a little sliding tab with an alignment mark on it that you can use to precisely compare the stab tips (for example). The beauty of this is that once you do one side, when you go to the other side you can see how much it's off by. Slide the tab to approximately split the difference and move the stab until the tip is on the arrow. This is WAY faster than measuring everything out if all you care about is symmetry (which I think is all you care about....I'm nowhere near doing this yet). When you think you got it, I'd maybe measure it one last time a bit more precisely just to make sure.
There are many different variations here to play with, but having SOMETHING on the string that's easily markable makes stuff like this a piece of cake.
edit: Re: tape measures.
Here's an interesting thing that I discovered recently. The little tab on the end...you know, the one that's never rivetted on right so that it's always really loose, is like that on purpose. It's so you can do inside and outside measurements. The amount it moves is exactly the thickness of the tab itself. Now that I figured it out, it seems obvious, but I wondered about it for years. Maybe I'm the only one that was confused by it but I figured I'd post it anyway just in case there's anyone else out there as scatterbrained as me.
