Pond liner tape is easy to talk about. 

Don't use it! While I haven't personally used it, I know people who have used it and they wish they wouldn't have.

You see the problem is the seam.  Water will find a way through a seam, one way or the other.  Nothing is more frustrating with a pond than a leak.  If you use pond liner tape, you more than likely will develop a leak.

You will be much better off by going with a full complete piece of pond liner which will give you peace of mind.

As far as calculations go for your pond liner once you have the hole dug is as follows: To figure out how big of a liner you will need, do these quick calculations.

Length of liner: Length of your pond plus 2 feet by the depth of your pond plus 3 feet. Width of your liner:  Width of your pond plus 2 feet by the depth of your pond plus 3 feet.  This will give you the size of the liner you will need for your pond that you have dug.

Just stay away from pond liner tape.

Having a good pond liner is very important and will last you for many years.  Try not to cut corners when getting your pond liner.  You won't regret it.

By having a good pond liner from the very start of your pond, you won't have to worry about using pond liner tape.  Just get one piece that's large enough to cover the entire area.