When doing lock mortises/latch drillings it is essential to get the mortise/drilling straight, and ideally very slightly oversize. Similarly it is essential to get the door drilling centres bang on - for which purpose a jig is the way to go - and to leave enough space to allow for bumps and humps on the lock/latch body, circlips , etc - again by drilling a slight amount oversize.
As an aside, these mechanisms with sleeve nuts have made it all but impossible to make door adjustments for tight doors by planing a bevel on the leading edges (as people of my generation were taught to do in college). The leading edge of the door now has to remain square (are your doors actually square?) if you are to avoid problems. If the door is tight you now have to take any easing off the hinge edge, square, and rerout the hinge recesses (because doing it by hand often won't get you there accuately, especislly with radius corner hinges). Not a complaint - just a comment
That is just perverse. The last two or three jobs I've been on we used commercial quality door ironmongery from firms like Arrone or NewStar. They aren't the best (but they aren't rubbish, either), but I find that the "drillings" (cut-outs) for the sleeve bolts on commercial ironmongery is almost always oval with, and not tight at all, which gives the installer a bit of leeway on the handles. When I install stuff I tend to drill my holes for the sleeve bolts a couple of millimetres oversize, as well, and I eyeball the handles from the end of the door to ensure that they are visually square to the door and don't end up one high and one low, which can also force you to install the sleeve bolts at an angle - and if you have to install the sleeve bolts at an angle then that is just wrong. Do these lock/nandle sets assemble "dty" out of the door? If not then you either need to change the locks or the handles as it just stores up problems for the future.
I also tend to pop a few countersunk screws through the base of the handle if they are supplied - but you'd be surprised at how many times the screws are maybe 0.5mm too long and can pinch the lock body, making the action "tight". A quick bit of work with a file to take the tip off generally sorts this out, but just in case I always have an extra supply of shorter screws (12, 15 and 16mm)
I'm sure that as a "lockie"
@JohnD will be able to quote the standard, but there is a recognised CE or BS standard for these, and if the supplier is even half decent they will work