Goal Seek on multiple cells (HELP)

So, I'm working in a telecom company in Postpaid section, we have recently changed our billing system and now we are facing problem with setting limits to customers.
for example : a customer requests us to set his monthly limit to 500. while entering the 500 in the limit area, the system allocates the limit in the below format { 500*10%(10% telecom tax) - 400(two months rental) } thus the system will only allow 50 limit per month for the number. [ 500*10%-400 = 50 ].
my question is ( how to set the exact limit as per customer request i.e. 500 per month instead of 50. I could Achieve this by Goal Seek but it can't be applied to several cells and I have thousands of numbers. attached is an example.