Straight edge ?

I like how the picture shows a rickenbacker bass!

 

I have one. Quality is fine, for a machined bar of aluminum. It's good for checking relief, but using a capo and fretting the string seems to work well, too. This one is a lot less expensive than the one from stew mac.

 

Uneeded. Just sight down the neck for nearly dead flat and check that theres barely any clearance from bottom of sting to top of fret at 12th fret with E string fretted at first and 24th fret. Adjust bridge for lowest action with no fretting out. If you get excess buzz at a couple frets after that you prob need a fret level job done by a qaulified pro luthier.

 

Yeah just fret the string at first and last frets and measure clearance at the 9th fret. Just enough for at least a business card to fit under. Adjust to taste and the frets will tell you if you need more or less relief to keep it buzz free. As dark storm said problem frets that buzz will require pro attention to get action as low as possible if that's what you want.

 

A good use for the straight edge is to adjust the neck dead straight for fret leveling. I agree that the string method works fine for adjusting neck relief.