Try the following:
LOOKUP(SUM([TOTAL Auto UNITS]), -[lag])
Here I've used a parameter [lag], but you could replace it with 1 to get t-1.
After entering the formula for the calculated field, you might want to click on the blue text "Default Table Calculation" in the upper right corner and select Compute Using > Month.
Lagging Example_jimw.twbx.zip 87.8 KB
Worked perfect for me...Thanks Jim.