@tonythm, @montylowe , @Anonymous , @Anonymous
So, just to sum this all up, and finally close the topic for future readers who find this by searching...
When working with Parameter, ModelParameters, UserParameters, etc., by code, the following guidelines are in effect:
To create new Text type parameters or new Boolean (Yes/No, True/False) type parameters, you need to create it using the 'Value' property of the parameter, and not by using its 'Expression' property.
To create new Numerical type parameters you can use either its 'Value' property or its 'Expression' property.
When creating new Numerical type parameters, if you use the 'Value' property, the result will be a simple numerical value, not an equation.
When creating new Numerical type parameters, if you use the 'Expression' property, you can specify an equation, which can include the names of other parameters, and that equation will be maintained within the parameter, or you can simply specify a simple numerical value instead of an equation, and it will also work.
Using the 'Expression' property of a numerical parameter, is the only way to maintain an equation within the parameter, instead of just a simple numerical value. Using the 'Value' property of a numerical parameter can only result in a simple numerical value, and can not store an equation.
If...Then statements and other non-mathematical logical code can not be entered into the expression of a parameter, because they are not purely numerical.
Any questions?
Wesley Crihfield

(Not an Autodesk Employee)