We have SLA model like this;

Customers can open ticket releated to their SLA packet, and SLA packet says that, for example this customer can open 30 tickets for 1 year, Customer has opened 10 ticket then we want to see that, "This customer can open 20 ticket from now"

How can we calculate and add field "This customer can open "X" number of ticket until from now". 

Is there any way to do this ?

Thanks in advance,

Sinan