To calculate LCG_min with respect to LCG it is possible to use the Change Field Values window

Click the Items menu and select Change Field Values...

Select the items in the item window you would like to do this calculation for.

1. In the Change Field Value window set Field to LCG_min.

2. Set the Operation to SQL Operation (SI units).

3. In the Value field type in the formula LCG - x. Where X are the length you would like to calculate LCG-min from LCG. 

4. When you click Apply the operation is executed for any items marked. 

To read more about the Change Field Value window check out the Online Help Manual.