As far as it is known, the percentage is not quite as set. Those with add or adhd do not as rule have spasm, tics or odd movements. This can actually be be an underlying secondary condition, Such as baseline tourettes, and there is a disorder that often accompanies ADHD. Many adhd sufferers show repetitive motion disorders as well. Not all kids with one automatically show up with the other.
Certain medication combination can exacerbate these motions. And certain medications can of course cause it.
Both are definitive brain disorders with differential diagnoses one can see if each is co dependant. That is if one is present is the other.
For years drugs used for ADHD and attention disorders were mono therapies. But as more comes out, secondary complications such as movements, vocalizations, staring might be indicative of something more or a build up level causing complications. Like too much Tegretol which is used to control seizures in epileptics, too much can actually cause seizures.
No drug has ever been studied in depth to show tic causation.
However a combination of Methelylphenidate and Clonidine have been used to control both ADHD and tics.

When my son who is now eighteen was first diagnosed 11 years ago, I researched on the internet to see if the tics and other jerky movements were part of the ADHD, and came up empty handed. It seemed while he was on medications for the ADHD it got worse, and it was worse on some medications than others were. He did the merry go round of those medications for 3 years until we took him off.

As you might know, there is ongoing research into this disorder and studies have shown that tics do occur. The more agitated the child is, the worse the tics are. I know it is hard to settle down a child when they are excited or angry but you will find once they are calmed down the tics are not so rampant.

About 10% to 35% of people with ADHD also have tics, such as spasms of the muscles of the face (facial tic), clearing the throat, sniffing, and excessive blinking. Stimulant medications for ADHD may make tics worse. Recent research found that atomoxetine (Brand name - Strattera) did not make tics worse.