Perspective and converging verticals

I'm currently in the process of shooting picture frames for a website and I need a little bit of advice as this isn't a standard product shoot. The way the pictures will be used is that users of the website will be able to pick a frame and a mount and put them together so they can choose exactly what they went.

Therefore the perspective of each picture needs to be exact which is where I am struggling as although the shots look straight there is actually a little bit of convergence of the verticals, usually only a few degrees but possibly enough to cause a problem when they go online. I guess what I'm talking about is lens distortion because obviously if I shot these with a 10mm lens then there would be quite severe distortion.

What I'm wondering is if there is a formula that takes into account things such as focal length, distance + anything else relevent which would give me the optimum focal lengh and distance to get rid of the convergence. I appreciate that perhaps the only way to do this would be to shoot with a 5x4 but that's not an option available so I'm hoping a better understanding of the maths and physics involved might be able to help.

Also is it possible that my camera simply isn't level enough with the subject so although it looks level it is infact perhaps pointing up a few degrees? I suspect that the solution is a combination of both but any advice would be greatly appreciated even if the only advice is to stop looking so hard at the final image as a few degrees difference is the best I'm going to get.

In order to avoid the convergence of parallel lines (such as walls of a building or railroad tracks), the film (or sensor) plane of the camera needs to be parallel to the two lines. In other words, when photographing a building the film/sensor plane needs to be absolutely vertical.

The way to choose the framing of a building in an image while keeping the film/sensor plane vertical is to use a camera with adjustable positioning of the lens relative to the film/sensor. A view camera is the best tool to use, but there are lenses for EOS cameras which can do some of what a view camera can. Look at the various Canon tilt-shift lenses such as this TS-E 24mm f/3.5L II.

Converging vertical lines comes from tilting the camera. The lens can make lines curve via barrel, pincushion, or wavy line distortion. The first two are easily correctable via software. The last is particular to ultra-wides, but I dunno how easy/difficult it is to correct. You might have to buy something like DXOMark if it's a problem.