[gobs]

Should 5% ones be used neat then?

In some recipes they are.

As for cucumbers, they can be bottled in salted water as well. Does not store more than a few months, though.

[WG.]

A formula ...

You need to know the starting strength of your vinegar = start
You need to know the desired finishing strength of the mix = finish

To figure out how many parts water to add, compute start divided by finish and then subtract 1.

Example :
If start = 5% (normal for UK) and you want to finish with 4%

5 / 4 = 1.25
1.25 - 1 = 0.25 which means add 250ml of water to each litre of vinegar.

[gobs]

That's great WG, and

this might also be of some help.