Laser pointers would work well. As far as exact amount, only you can tell. It varies w the room, the speakers, the distance away from the wall, your personal priorities, etc.
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The correct toe-in is dependent on a wide range of considerations -- personal preference, room size and shape, distance from listener, speaker design and voicing, etc. As one of the last steps in setting up a speaker, I personally experiment with the amount of toe-in.
First, start with the manufacturer's recommendation. The speaker is "voiced" to some extent to sound its best (in the manufacturer's opinion) on a certain axis. The frequency response of speakers vary greatly depending on the toe-in angle (because dispersion of drivers are not perfectly uniform, driver-to-driver interactions lead to cancellation or boosting of frequencies where the waves from the different drivers blend, etc). This recommendation is merely the manufacturers best guess at what sounds good in a majority of circumstance, but, someone serious about sound will never automatically stick with the recommendation.
A lot of toe-in will help to reduce reflections from a nearby wall. This may help with imaging accuracy, but again, this is not always helpful. Speakers pointing nearly directly at the listener also tends to firm up the center image and make the position of centered vocalists seem much more clearly defined. But, the cost is often a narrower sounding soundstage and a loss of some sense of hall acoustics and sound enveloping the listener (everthing is more tightly focussed in front).
I generally don't like severe toe-in that crosses in front of the listener. But, it can be useful where you need a sweet spot wide enough for more than one listener. With severe toe-in, the listener who is closer to the right speaker will actually be more on axis with the left speaker which somewhat compensates for the closer proximity to the right speaker.
Again, experiment. One slight issue is that big changes in angle are actually change other parameters slightly, so you may also have to fiddle a bit with general placement and backwards tilt to get the optimal combination.
Larryi covers this quite well. I would only add that Speaker designers assume you can/will set up their speakers in situations that complement their design. For example, one famous maker recommends facing the speakers straight ahead (they had a very hot on-axis response, and just sounded much better off axis) but if placed near an untreated sidewall they sounded especially harsh. They benefitted by crossing the axis well in front of the listener position in many set ups.
Another thing to consider is that extreme toe-in changes all of the reflection points in the room. For example, the left side wall will now become more important in the sound from the right speaker because you have strong on-axis sound bouncing off it. Ditto ceiling and floor. In some rooms this is a good thing, in others not.
FWIW, in my room, crossing the speaker axis in front of my listening chair has allowed me to get a wider speaker spread and has eliminated some distortions brought by room created distortions from reflections without having to resort to a lot of audiophile acoustic treatments and the consequencial WAF issues.
But, if side walls are not an issue, many speakers facing forward will sound much more expansive - soundstage wise.
Anyway, just fiddle around and sometime in the next year or so you will find the perfect toe in and speaker set up. Have fun.
The method described is essentially the "Sumiko Method" taught to dealers of Sonus Faber and Vienna Acoustics" speakers. I have helped a dealer set up several systems using this method.
This method emphasizes getting the bass response right. What is amazing about this method is that even very tiny movements (1/2 inch makes a difference) into and out of bass nodes has a noticeable impact, and everyone listening will hear the same subjective change no matter where they are located in the room. I've heard, and agreed with others on the benefit of a movement on bass response while standing next to the other speaker (the one not moving). Toe-in is one of the last items that is addressed using this method.
i own a pair of magnepan 1.6 and recently acquired a pair of quads unlimited quad 57s.
i am the speakers toward the side walls, which are treated with a sound-absorbing material, in order to change the frequency response. since there is some absorption of the upper frequencies, i find the overall frequency response, closer to a balanced spectrum. of course, taste and preference are the issue here. try it and see what you hear. you can always move the speakers back to their former position.
Clio09, Mr T correctly makes a case for trying something different, and he is also right that you will hear a (major) change in spectrum balance, i.e. substantial roll of of high's and mid's, but it will also dull the signals which are so important in creating the information essential to resolution and imaging. Try it for sure, just don't spend any money..........:-)