Here is an example... this is a threshold on disk % usage, depending on the size of the disk.
If disk is smaller then 1GB - 80% threshold.
1GB-1TB - 90% threshold.
1TB+ - 95% threshold.

storageSize = AgentReadParameter($node, "FileSystem.Total(" . $dci->instance . ")");
if (storageSize == null)
  return false;

if (storageSize < 1099511627776) {
  // for storageSize under 1TB
  if (storageSize < 1073741824) {
    // for storageSize under 1GB, 80%
    usageThreshold  = 80;
  } else {
    // for storageSize between 1GB and 1TB, 90%
    usageThreshold  = 90;
  }
} else {
  // for storageSize above 1TB, 95% threshold
  usageThreshold  = 95;
}

if ($1 > usageThreshold)
  return true;

return false;

if (current dci >= 100 and secondary dci is true)
{

trigger event

}

else
{
keep on going
}

* Does the threshold script need to return a boolean  or integer to be evaluated

Im almost succesfull at what im trying to achieve but im stuck on figuring out a way to reset values after the polling interval  I have four DCI's that collect and increment the values every minute and using your code created an internal DCI which will sum those values every 15min and i want to trigger an alarm based on the totals of the internal DCI  but how do I reset the values  the DCI's  after every poll interval instead of permanently incrementing

Then make the threshold on that DCI.

Sample script attached below.
Save it to script library and call it from the script DCI.

val1 = GetDCIValueByDescription($node, "DCI1 description");
val2 = GetDCIValueByDescription($node, "DCI2 description");
val3 = GetDCIValueByDescription($node, "DCI3 description");
val4 = GetDCIValueByDescription($node, "DCI4 description");

if (val1 == null || val2 == null || val3 == null || val4 == null)
  return 0;

return val1 + val2 + val3 + val4;