If you remove the numbers after Calculate in the Implements directive and the function definition, your code should work.

Option Explicit
 
Implements ICustomFPScriptFunctionCalculate
 
Private Function ICustomFPScriptFunctionCalculate_Calculate(SafeArrayOfArguments() As Variant)
 
	ICustomFPScriptFunctionCalculate_Calculate = SafeArrayOfArguments()
 
End Function