How to set priority of a bug in a web application?

Software bugs are un-avoidable. No one can deliver bug free system at first stage. It means some how we need to deal with bugs. Here I have written some guideline to set bug priority in web application. One can use it for other applications too.

Consider a small web application where user can register and upload photos & videos after log in. I have used this example to visualize various conditions as below.
Here P1, P2, P3, P4 & P5 are priorities.

P1: Blocking, High level importance & Can occur during Testing/QA phase.
e.g.
(1) Development is done, but QA is not able to start testing as he is getting blank screen due to improper DB connection.
(2) User is not able to register or sign in.

P2: Blocking, Low level importance & Can occur during Testing/QA phase.
e.g. User is able to login but there is some problem in photo upload feature and user is not able to upload photo.

P3: Not Blocking, High level importance & Can occur during Development phase as well as Testing/QA phase.
e.g.
(1) Normally when ever you start developing some module, by default you can set priority to P3 for all your tasks.
(2) Forgot password feature is not working.

P4: Not Blocking, Low level importance & Can occur during Development phase as well as Testing/QA phase.
e.g.
(1) Design related issues. Thumbnails are not looking good for photos in photo album.
(2) Browser issues. Sign Up page is looking great in Firefox but not aligned well in IE & Safari.

P5: Not Blocking & Can occur during Development phase as well as Testing/QA phase.
e.g. Text changes. Error message is not proper / some spelling mistakes

You can deliver your project successfully if it passes following criteria.
Total P1 Open should be 0
Total P2 Open should be 0
Total P3 Open should be 0
Total P4 Open should be 5 (You can change this no.)
Total P5 Open should be 5 (You can change this no.)