When setting up a tent and staking it out, what is the proper tension for the lines? I generally just make them arbitrarily tight, but have no idea how that would perform in high wind or other conditions.
There's no hard and fast rule for specific tensions that I know of, mainly because that will vary slightly depending on the conditions. As an average rule I tend to make them taught, but not to the point where they're pulling on the pegs.
There's a couple of scenarios where I tend to slacken them off a bit though:
When the pegs can't be fully pushed into the ground for whatever reason. In theory they should always go all the way in but sometimes if you don't have gear up to the job of penetrating the ground, they won't be as secure as they could be. If this is the case you generally want them tight enough for them not to fall off the peg, but other than that a bit of give is sometimes good.
In high wind conditions I tend to slacken them off a bit just to lessen the chance of the wind pulling them too tight suddenly and snapping them. In these conditions though I make sure I've got some paracord or similar so that any repairs that need to be made can be quickly. Helps to brush up on your knots!
Other guidelines are much more important though - things like having your pegs in at a slight angle away from the tent, making the lines nice and long and spreading them out well. I find those 3 things generally much more important than how tight the lines end up being.
When I was in the scouts a few years ago we used to use patrol tents with natural fibre guy ropes. We were always taught to slacken the guys off when it started to rain because the ropes shrink. Sure enough one patrol didn't do this and the 2" diameter ridge pole snapped leaving some very unhappy campers.
I realise that most tents these days don't use natural fibre guy ropes but I think it's still worth being aware of.