Use TimeSpan.FromTicks() to calculate default TargetElapsedTime

Fixes issues #1184, #1085
TimeSpan.FromSeconds() has an accuracy of 3 decimal places only.  TimeSpan.FromTicks() is more accurate, resulting in the expected target elapsed time of 0.0166667 rather than 0.017.

Reference: http://msdn.microsoft.com/en-us/library/system.timespan.fromticks.aspx