The chalk line is a great idea for marking the fuse. I've found that a flexible tape measure (like a tailor/seamstress would use) is invaluable when covering.

Using the turtle deck as an example, here's what I do:

1. Mark the horizontal line down the fuse.

2. Use the flexible tape to measure the distance over the top of the fuse. The number of measurements depends on the shape of the model. Sometimes you can get away with one measurement at the rear-most part of the fuse and one right behind the canopy. Sometimes, I'll take measurements in 3" to 6" increments. I almost always put a seam in the middle of the canopy. Use the same technique for the front.

3. I then use those measurements to make a paper template of the piece. That way I can test fit and tweak on paper which is much less expensive than covering.

4. Once I'm satisfied with the fit, I trace the shape onto the covering and cut it out.

5. Then, what MinFlyer said. Once the seams are tacked down well, you should have a piece that requires very little shrinkage.

Here's a Goldberg Sukhoi that I covered a few years back. Wish I had thought of using a chalk line though!

This technique also helps you get the most out of your covering. If you make templates for the major pieces before actually cutting the covering, you can lay them out on the roll of covering to help reduce waste.