I just want to cast well and not spook fish.
This statement is the bottom line when it comes to leaders! Those "rules" are not really rules at all, just generalizations, and are in most cases a good starting place.
As Milt said, there are no standards, technology has changed everything. Advances in both rod materials & line materials allow casting that could not be achieved many years ago. Today's leader materials can be had in many degree's of stiffness, and diameter, and diameter to strength ratio's are much better today that they used to be.
However, as these guys have said, what leader you use will depend on a few variables. The fly, how wind resistant it is, how heavy it is, it's type (wet/nymph, dry, popper, streamer, etc.) the target species, and the rod/line wt combination you're using. Plus, to some extent the weather conditions too.
I've used the rod length to leader length + 1 ft ratio quite a bit. It works fine with smaller size flies, (14 to 8) what most folks use for trout or panfish, as long as the leader is sized properly for the line wt. However, to cast some flies well, and get the fly to land where the tippet is extended properly this ratio may have to be adjusted, either slightly shorter, or longer. For example, add barbell eyes, beads or cones to a fly, the weight is significantly changed, so a shorter leader/tippet combination may work better. Sometimes even shorter than the rod length.
Generally, the heavier the fly, the shorter the leader, and often the heavier the tippet needs to be. Of course, most leaders will cast a range of flies well, it's when you attempt to change flies & go from one extreme to another that usually creates issues. For example going from a size 16 dry fly to a size 10 bead head nymph, the leader that turns over the dry fly well, may not do so for the heavier nymph.
When selecting your leader, you should use the shortest one that will properly turn over the fly you select, and not spook the fish. Long leaders, over 10 ft, are sometimes necessary, but the longer a leader is the more difficult it is to control on the water, especially in moving water. For example, if your using a 12 or 13 ft leader & catching fish, that's great, but if you can do the same in the same conditions with a 9 ft leader, then the 9 ft would be a better choice. Some folks have success with long leaders, but they're usually only necessary in special situations.
caseywise, if you start with a leader length using that ratio, it will work. Just keep in mind that adjustment may be necessary for the variables I mentioned!