Is this rule correct?

[improvpwnd]

If you change strings to a lower gauge, you should add relief to your neck because there is now less tension. I recently went from (.45, .65, .90, .110) to (.45, .65, .85, .105) and without any adjustments there was a lot of buzz...so I loosened the truss rod 1/4 turn and I am going to wait till tomorrow to see what happens. I'm thinking I will have to lower the bridge too to get the action like it was before the string change.. whatchall think? Am I doing the right thing?

[cerrem]

In "theory" if all other variables remain constant, then yes you will have less string tension, therefore relief would be in order..
This is assuming you are using the same brand and type of strings...For example, lets say you went from soft nickel set to stainless...then it could be another matter all together...
The neck adjustment is not a guessing game...It's as simple as looking at it and seeing where it is... Look at it from both sides of the neck... Press the low E string at 1st fret and then at the 15th fret....Take a GOOD look from the side of the neck..also do this on the G-string.... You are looking for the spacing between the string and frets...You want it to be as uniform as possible...but you want some small relief for normal string vibration..at the deepest point of the relief, I like to have it the spacing of a credit-card on a bass....some like it straighter like the thickness of a business card.. But that does not always produce the best tone...Remember you don't want it 100% straight, you need a little relief...
You want to look at both side of the neck because sometimes certain gauges on certain basses cause different pulling...A bass with a very thing neck is relying heavily on the truss rod..while a fat, thick laminated neck may be more robust and less likely to behave wacky on certain gauges..Try fretting a note with left hand on the 5th fret and also on the 12th fret..try plucking behind the string so see if you get a clear note with your right hand...do this for each string..This is an indication if nut depth is good or not....
If you have too much relief, then you will get lots of fret buzz on notes played in the middle of the neck and vertually no buzz on notes played after the 14th fret...
Once you have the neck set nicely, then adjust the saddle heights for where you like them...try to maintain some contour to the neck...

Best Regards
Chris

[tappingtrance]

Also rememebr when you change gauges your stirng acion will change - changing gauges does require adjustments of al sorts.

[fretlessrock]

What I do is measure my setup before I do the string change. Then I can adjust the bass to get the new strings into that same ballpark. That assumes that I was happy with the setup to start with.

The reason that you might add relief is that the lighter strings are inducing less relief, so you need less truss tension. I don't worry about how many turns or whatever as long as the truss is healthy. I would have a measurement or even a good eyeball guess, and then adjust the truss to get back into the ballpark with the new set. Either way, stick to one adjustment at a time. If you want to get into the saddle adjustment then do it after you have the relief close. That way you can set the intonation and saddle heights at the same time and not need to do a lot of tweaking.