PR Help says the following: 

"Last check amount

Displays the amount of the last check printed for this employee, which is automatically maintained by Payroll. Do not change this amount unless you are setting up an employee for the first time, or you are sure a change is required. The last check amount is not updated by Void Checks; it stores the amount of the last check printed even if that check was subsequently voided"

As far as I know, you cannot make another field automatically update with the Net Direct Deposit amount.  However, you could submit an enhancement request to Sage - under Help - Submit Product Idea. 

Turns out - it had been submitted so I voted for that request!  THanks for the feedback!

I always wondered why this didn't happen as well.  So I voted for the enhancement as well.  It took a bit to find it.  If anyone else wants to vote for this here's the current enhancement to search for:  

Payroll - change employee set up misc info tab last cheque amount to last payment amount