The two things I found most helpful were inexpensive laser measures and casters. My listening chair is against a wall so I put a laser measure on the top of each speaker aimed at a single spot where my head would be, but you can do it with one and some blue painter's tape. You can also measure the distance between speakers. But to achieve exact distance and angle I found having the speaker on casters let me get the placement 'dead on balls accurate' (it's an industry term) for distance and aiming. My casters are attached to a metal sound deadening plate, but by matching the speaker spike thread to the caster thread larger heavier speakers in particular will be easier to 'dial in' precisely. A dense rubber caster wheel will probably isolate the speaker from floor vibrations as well as anything else. This system beats a tape measure or string or any other guessing.