Ancient German measurement GootNtight.
Lightly snug injector lines, start the engine, tighten each just enough to stop any leaks, no tighter...

Test drive, and check for leaks, re-tighten as needed..

This is an art master mechanics develop after tightening thousands of bolts.

Note:
On injector lines, you must watch the mounting angles.
If the line clamps or steel line where tweaked, you will need to install them one at a time, and re-align each line with the injector and injection pump fitting.

What would be the reasoning behind this as it seems to be counter-intuitive to me? Are the injector lines particularly susceptible to damage? Or is there another reason?

I hope I don't have to wait until I tighten thousands of bolts to find the answer myself.

__________________
1985 300D Turbo
"Evolution is God's way of giving upgrades" Francis Collins

The ball on the end of the line and the socket of the injector are of different hardness steel. They are an "interference" fit.

You are attempting to create a seal between the mating surfaces by compression. Cranking down on the line nuts can cause scratches on the sealing surfaces which result in leaks, which results in cranking down the nuts tighter, which leads to.....

Eventually you stretch or warp the softer metal nuts and they don't provide the proper tightening torque.

Examine some old injector lines. You'll see a "cup" shape worn into them. This is from either the lines being too loose and allowing the ball to rattle in the socket or from being crushed into the much harder injector receiving socket.

Thanks. Now that I know the reason, I'm more likely to remember to do it the right way.

__________________
1985 300D Turbo
"Evolution is God's way of giving upgrades" Francis Collins