Sorry to jump on an old thread, but as for altering string length - that can be done accurately, quite easily, there probably is a full description on the site somewhere but I've just searched and failed to find it.
As described above, under tension with staked lines, work out how much you need to adjust a line by, measure that distance with a ruler.
Then, before you untie any knots - mark where the actual end of the line currently reaches to along the line.
Not sure if ascii art works ok here, but roughly - main line comes in from top left, makes a loop, and then comes back through a knot onto itself:
^Mark this spot on main line
Untie your knot - should be a simple overhand knot, move the end of the line down the main line *double* the distance you'd measured, adjust the sleeve (dont remove it!) so it's central to the new loop, and tie an overhand knot again to secure it.
Then compare them all again to check ;-)