A fluid and flowing pendulum motion generally ensures a smooth ball toss. The speed of the backswing greatly determines, the ideal height and timing of the toss.
Players usually toss the ball approximately 20 - 40 cm above the contact point, in other words, the ball descends slightly to contact. The server jumps upwards against the falling ball. This may be slightly different for players with shorter and quicker backswings.
Video of Rafael Nadal's Ball Toss
[av_video src='https://vimeo.com/97308609' format='16-9' width='16' height='9' av_uid='av-9wbegvy']
Video of Andy Murray's Ball Toss
[av_video src='https://vimeo.com/97308608' format='16-9' width='16' height='9' av_uid='av-7rn8we6']
Rafael Nadal - Height: 6'1" (185 cm). Height of ball toss: 321 cm.
Nadal tosses the ball 0.38 m over the contact point (at 2.83 m).
Andy Murray Height: 6'3" (190 cm). Height of ball toss 366 cm.
Murray tosses the ball 0.80 m over the contact point (at 2.86 m).