G'day Mate,
It is no good using string, stretches too much, but anything that is solid will do, I use a straight edge rule, but a long piece of dowel or wood will do, just mark it, & then move to the other side & adjust to suit. the horizontal stabilizer can be a bit out, but the fin has to be as straight as you can make it. It is also important to make sure the tail & the wing are aligned, Parallel to each other, so look from the tail towards the wing hold the tail up so you can look straight down the centre line of the fuselage & make sure they are lined up, if they are out a bit adjust to suit.

Good Flyin Mate,
Allan.