In this case you have to reduce the output shown in the card (see page 10 of the rulebook), Not the final output. For example, if you have Bank02 with a banker (necessary employee) and two workers +1 (bonus employees) you normally produce 5x3 = 15 credits.
With this event you produce 5x2 = 10 credits instead. I hope this will help.
The output is the number shown in the card (bottom right). The multiplier are the number inside the multiplier icon (see page 12 in the manual) plus the extra multipliers generated by the employees. In this example, the number 5 is the multiplier, the number 3 is the output ant the number 15 is the final production.
Yes, this is the correct way to play this card. It should probably be posted as an FAQ.