I'd guess that the recommendations are so varied because there are many ways to deal with this rather large, fast growing evergreen. Do you intend to embark on a shearing program with your trees, and grow them as a manageable hedge? Or will you allow them to attain their full-size potential, suitable only if you have PLENTY of room to spare?

No desire to have to shear them a few times a year? Then plant them at 10 foot centers. If you an extremely wide border in which to plant them, you won't need to stick to the straight line planting, but 8-10 foot centers would still be the closest you should install them. As they mature, their branches will grow together but they won't overpower each other.

You should also know that these trees are considered rather short lived, more so if exposed to any of the fairly serious diseases that can affect them.

Here is a link that might be useful: FYI

I'd go with a zig zag pattern. You'll find that none of your lelands will grow at the same rate and that this will be less apparent if they're not in a straight row. Be aware that they get knocked over pretty easy by hurricanes if you're in a coastal area.

My favorite tree for privacy screens is magnolia grandiflora if you have the room and the "Little Gem" variety if you don't. With water, fertilizer and good soil they grow faster than you think.

Cryptomeria is also good. Better form and a bit slower growing than leylands.

I have read in several places that Leylands don't have much in the way of environmental contribution. Leylands are actually a cross of two other kinds of trees. They don't have berries they aren't a favorite of some important creatures, etc. There was a suggestion to try Thuja Giants (which, I believe, are a cultivar of Western Red Cedar). They also grow very quickly and have a much more positive impact on the environment. Eastern Red Cedars are available at the national arbor day foundation website. $10 annual membership fee gets you 10 small free trees, ready for planting. They also grow quickly and are pretty easy to grow, from what I have read. I ordered some of those and planted them in the fall. It's easy on the budget to get trees this way too! :-)

Here is a link that might be useful: National Arbor Day Foundation

Hi-I have many Leylands, 15 are a screen in my front yard. I planted them 15 years ago 8 feet apart, in a straight row because I didn't have alot of space. I wish that I had planted them 10 to 12 feet apart. When they mature, the inside of the tree, like many evergreens, shed their foilage, so your left with foliage only on the outside of the tree. Also, beware of bag worms. I needed to have them professionaly sprayed because of their size. Now I am sorry to say, they have scale, which is killing them. I have had them sprayed last fall and a second treatment is due this spring to try to save them. In retrospect, I should have planted something completely different. Maybe a large bush like weigela which is not evergreen, but grows densly and is beautiful in the spring. Look into other options.