To view the Capital Allowances Calculator you click on this link. Alternatively you can also follow the navigation as shown below. 

Navigation: Self Assessment >> SA100 >> Client Specific >> Edit >> Supplementary Pages >> Net profit(loss), Tax, allowances and Taxable profit(loss) >> Capital allowance 

Video Tutorial:

After you have navigated your way to the client specific SA100 dashboard, either create a new SA100 Return which will appear in the SA100 Forms or select the 'Edit' from the 'Action' drop down of an existing one. 

Enter the 'Supplementary Pages' tab.

Select the 'Net profit(loss), Tax, allowances and Taxable profit(loss)' link which will open the prompt below.

Enter the 'Capital allowance' tab.

Fill in all the details as required and save the form.