How will you calculate the total SLA for both INC and INC task..

Total time for INC task sLA should be within INC SLA.  So, if INC has 5 days SLA better would be to have INC task SLA to may be 2 or 3 days.

How will you handle priority changes from INC to INC task.. 

Priority should have any impacton INC Task. It should be independant of Priority & has been the case with my different engagements until now.

Simple ex - Lets say total SLA is 1 hours. You might create an INC task at 1st min itself, or you can create it at 59th min. Theoretically it sounds simple, but practically how its done/how others are doing is what I am trying to understand.

As said it depends on use case. If you have P1 obviously you are not going to create a INC Task. There will be a call setup where all members will join & look for a fix. Ultimately, there has to be one team who will be responsible to take ownership or held accountable for issue. If there is a different team involved this would tend that issue is not with one system but with multiple.