The width of the core is related to the width of the finished board. What are your base/board dimensions? Will you use sidewalls and or tip and tail spacers?
You should have a base dimension ready and this well tell you. This is pretty basic.
Make it .5cm bigger all the way around and you are good.
Use snowcad(www.grafsnowboards.com). It allows you to create your nose and tail widths and then it calculates your waist width. I typically use a 300 mm to 310 mm nose and tail width but I like wider boards.
