NX How to create associative expressions with drawing sheet size?

Currently there is no way to create associative expressions which hold the length and height values from a specific drawing sheet size.
There is an existing Enhancement Request ER-1879756 which asks for the ability to control the sheet size by expressions.
But this document will focus on the reporting approach FROM a sheet TO an expression, for example to use them later for 
- associative positioning of annotations based on the sheet extensions
- scaling custom symbols by sheet extension fractions
etc.

Solution

As a prerequisite, the suggested solution requires a Knowledge Fusion Author license to adopt an existing Drawing Sheet object and to create KF rule based expressions with associative values from that Sheet rule.

As part of subsequent workflows it sounds reasonable to add drawing sheet size expressions in a drawing template to inherit them automatically when creating a new drawing part using File->New.

1. open the desired drawing (template) part
2. ensure to turn on the Knowledge Fusion application to display the Knowledge Fusion Navigator (KNT)
3. Developer Tab -> Knowledge Fusion Group -> Adopt Existing Object
4. select the desired drawing sheet from the Drawing node in the Part Navigator and confirm

In the existing Drawing Templates provided with NX, there is usually only one Drawing Sheet "Sheet 1" but it works the same way for any other sheet.

5. in KNT on the root node, right click MB3 -> Add Child Rule...

6. instantiate a child of the ug_expression class and reference the adopted sheet values, e.g.
6a. Name: SheetHeightExp
6b. Class: ug_expression
6c. Parameter Name: "DwgHeight"
6d. Parameter Value: Sheet_1:Height:
6e. OK

7. repeat the steps to create another child rule for the length value, e.g.
7a. Name: SheetLengthExp
7b. Class: ug_expression
7c. Parameter Name: "DwgLength"
7d. Parameter Value: Sheet_1:Length:
7e. OK

8. confirm the User Expression in the Knowledge Fusion and Part Navigator