Click the "Parts" link in the "Measurement Actions" menu.

Click the "+Part" button, then the desired item type.

Fill the "Part" form and click "Add"

Notice the "Formula" field (4), where you can create your own Excel style formulas referencing the "count" value of the measurement. In this case, "count" = 7, hence count*10 = 70. This works for any Measurement Type, each one having its own Supported References ("len" for length, "area" for Area, etc).

You can add as many Parts as you like and these will be shown in your Quantities page. 

You can also edit the part details and formula by either clicking the "formula" itself:

Or clicking the "Edit" button:

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