The shaft being 50mm in diameter means the circumference of the SHAFT is 157.08mm.
The packing has to occupy the space between the shaft and the housing.
Given that the packing is 8mm, the actual length of the packing at the housing side would be about 207mm
If you cut your packing to 157.08 mm (good luck with that) I think you'll find it a bit short.
The BEST way to do it is to wrap the packing around the shaft (or some other 50mm mandrill) and cut through it diagonally.
That way you get an almost perfect fit which will completely seal when it is compressed by the nut bearing down on it.