Calculating Lengths For Ganglines-
There's a lot of variability in what you see in line lengths, from musher to musher and manufacturer to manufacturer.  It's not so much the actual lengths that are critical as it is the PROPORTION of the lengths of the centerline, tug line, neck line and overall harness length.  So, here's some points as I know them.

Tuglines-

I see tug lengths listed from 40" to about 55".  The shorter the tug, the more the diagonal angle of pull from the centerline to the front of the harness; the greater the angle, the more the distortion of how the harness sits on the dog's body.  The "pull pressure" is trying to create a straight line.  Longer lines have a more gradual angle of distortion and will not tend of "offset" the harness as much.  However, the longer each section is, the longer your overall team length is; depending on the style of running and the types of trail you have, too much length in front of the sled may not be an advantage.  Choosing to have a longer tug length for the wheelers gives a little more distance between the front of the sled/cart; several reasons for this choice.  1.)  It gives more "error" time if the drivers responses aren't as quick as they could be. 2.)  In addition, having longer wheeler tugs gives a more gradual UPWARD angle from the sled to the harness tug, thus DECREASING downward pressure on the rear of the dog.  It is desirable to add this length to the tug instead of extending the section between the sled and the back of the wheelers tugline as this maintains better responsiveness to the power of the wheel dogs, better steering and less "whiplash" effect.  This is important on a sled; however, when I am running a cart or the quad, I do use a bungeed extender from the front of the rig to the back of the wheel section since most of my control is coming from a steering wheel in this case.

Neckline lengths also vary according to the whims of the musher.  Ranges from 7" to 12" including the snap are common;  the shorter lengths keep the dog closer to the center line, longer ones allow more lateral range.  Tight, narrow or windy trails are times when "close in" can be an advantage; rough, uneven trails where the dog may need to vary the track without jerking on it's neighbor are where longer necklines are good.

Harnesses should be of a standard OVERALL length.  This means the length from the top of the neck (withers) opening to the back of the tug loop.  Smaller harness sizes should have longer tug loops and larger harnesses have shorter tugs, to keep the overall length about the same.  A range of around 29"-32" is what I have seen in measuring different brands of harnesses.  It isn't important what your "standard" is; the consistency of length in all of your harnesses is the thing to check.  The TAIGA's that I use run about 31" in length (withers to end of tug loop).

Spacers, when needed, I like to be about 12" long.  I use them in between gangline sections when I am concerned with the possibility of a dog chewing the tug in front of them.  My centerlines are cabled, but tugs aren't.

Centerlines- So, now you take your chosen tug length (including snaps), add your "overall" standard harness length and your neckline length (including snaps)......that is the length that your centerline sections need to be.  Be sure to account for any shortening of tug and neck length in attaching to the centerline.  Also, as you calculate your centerline lengths, remember to add any length BEHIND AND FORWARD of the tug/neck attachment points, ie. if they are resting at the back of a joining loop.  I make about a 4" loop at both ends of my centerline sections and they join by interlocking, making them easily modified.  Therefore, the necklines sit at the back of the forward loop while the tugs sit at the front of the rearmost loop.  I have to remember to add the "loop" length to the centerline measurements.  I like making individual centerline sections, tugs, necklines and spacers that can be taken apart when needed for repairs, changing team size, etc.

If you choose to have some sections a different length than others, you might use a different color rope for the centerline, making them easily identified.  Same thing with different length tuglines.

So, it ultimately is the length of gap between the tug and neckline that is the most important factor.  Too short a length, the dog will be forward too much and tend to get it's legs over the neckline.  Too long a length, the dog will be "strung out" too tightly and believe me, they don't appreciate that!   If you routinely run with other teams, it helps to coordinate your ganglines in case you want to swap dogs on a run or if you need an emergency spare part....they will have what you need or vice versa!  

Hope that is helpful.  You can use this info to make new ganglines or to check the balance on your existing ones and make adjustments.