Okay, implemented it how it *should* be done It is not using ReturnIDs in this example anymore:

Calculator 3 example

Btw if you compare examples you will find out custom class version is smaller than ReturnID version although more typing is needed. In addition I've created two other examples how encapsulated custom class can be utilized:

Calculator 4 example

Screenshot for Example 4:
Calculator4.png

And finally I put 100 calculators in one window:
Calculator 5 example

And screenshot for Example 5:
Calculator5.png