Based on the above specifications take the following steps:

Check the sales tax option    
If you do not want sales tax added to materials skip this step.

Type in your tax rate 
 If you do not want sales tax added to materials skip this step.
 At this point the  program has calculated your materials including sales tax.
 Your material price and total price per square in Total Price Per Square have been updated.

Click the roof pitch drop down list, select 6/12 Pitch  
The program looks at your Install roofing per square and updates all the labor prices based on 6/12 pitch for each material type in total price per square. All the columns in Total price Per Square are then totaled.

Check the tear-off option   

Click the Layers drop down list, select 2 layers   
  The program  gets your roof removal price by looking at your Roof Removal Prices for 6/12 pitch under 2 layers and copies this price to total price per square to the Roof Removal Row for all materials. All the columns are totaled.

Click the Add sheathing option    

The program looks at your OSB price       and multiplies it by 3.13

  The program adds this price to the OSB material row in Material Price. All these columns are then totaled.
These prices are copied to Total Price Per Square Material-loaded per sq row.
The program then looks at your Labor Grid under the  Lay OSB column , then adds this price to your labor to install roofing. This price is added to the Total Price Per Square, Install roofing row in all columns. All the columns in Total price Per Square are then totaled.