How to Change 2nd Parameter Value Based on 1st Parameter Selection

Hello everyone :)

I have 2 parameters (non queried):

Parameter 1 has 2 options: "A" and "B"

I want the default value in Parameter 2 to change based on the selection in Parameter 1. I've tried setting the default value as the following:

=IIF(Parameters.Parameter1.value="A",20,40)

This sort of works in that when the user first chooses A or B from Parameter 1, Parameter 2 correctly changes to 20 or 40 respectively. But it only works the first time, when the user changes their selection say from A to B then Parameter 2 does not update.

I basically need Parameter 2 to change if the user has changed Parameter 1, but they can over type with a new value, just when they make a selection in Parameter 1 I want a default suggestion to populate in Parameter 2.

Many thanks to any help in advance!