I don't know if you tested that but I noticed a problem with your math, you don't want to subtract $ctime from $uptime in the first part of the math. You are also missing a ) at the end of your $calc statement.
Also this will provide incorrect results if you use $uptime. Because if you load this after mirc has been running the uptime will be larger than the time the script has been running. Which will result in an uptime of over 100% which is incorrect.
Example of loading script and showing output:
<~pballz> Uptime Percentage: 163157.5% since Tue Nov 01 17:59:08 2011