Can anybody explain me runner & gate calculation for a plastic moud in Solid Works with a simple example?

Well, it is pretty a complex calculation but what most mold designers like me do is to use a set of standard tables to define all parameters.

First, depending on the shape of your product you will need to select the injection point and gate type.
Second, depending on the mass of the product you can define the size of the gate.
Finally, depending on the distribution of the mold cavities you define the runner shapes and sizes.

If you send me your model I can give you a more specific answer.

I hope you can read this web: www.xmhuaerjia.en.alibaba.com, they are full of plastic injection moulds experiences.