How much of my spline should be showing?
More specifically people will ask "How much of the blue should be showing?". The blue doesn't matter, it doesn't always start in the same spot so you should really be looking at how much spline is showing. The answer though depends on the type of spline you have. For Tom Wood's drive shafts there are mainly two types of spline; the S spline and the X spline. If you have the S spline, there should be about 1.5" of the smooth bar showing. For the X spline you'd measure from the dust cap to the center of the weld, this should be around 3 1/4". As long as you are within a half an inch or so of these numbers, consider your shaft a good fit. If the amount of spline you have visible is much more or less than what is recommended, you may need to have your shaft lengthened or shortened.
Below you can see what a shaft built with our X spline should look like when installed in the vehicle. The second photo is a side by side to give you an idea of how much spline would still be inside the slip yoke under normal operating conditions.
In the above photo, you can see the difference between the total length of the spline and the usable length. For safe and stable operation, the slip yoke and spline should maintain at least 2 inches of engagement at all times. That means that however long your spline is, you can subtract two inches from that to get your usable range of travel. You can see in the above photo that usable range is in red. The section shown in blue needs to remain inside the slip yoke at all times, even under full axle droop.