I have done a couple of my own mat boards.
If im guessing right......you are wanting the one mat board with 5 windows?
Basicly ...... what I did when I used some mat boards to frame some of my art work........ altho mine were single images..so it may vary a little for you.
Cut a back board for the picture to sit on, with the picture in the middle of the cut you should have at least a 2cm boarder around it of extra board.
Then cut another board...this will be the frame. This board should be at least say 3cm extra around if the picture is placed on it. Altho a important thing to remember is which ever strip that's going at the bottom is ALWAYS more. Always. So it should be say 4cm at the bottom, and 3cm the rest of the way around. Its to do with grounding the whole image and such.
Then measure your image/ text accordingly to make the window on the mat board. Remembering to leave a 3cm boarder/ 4cm at the bottom.
Also if you want a white boarder between the image and the mat frame, when measureing up for the frame and image size add about 5mm to each side.
And a hint for when it comes to cutting the window, put some small cuts in all the corners before you start the sides. This will stop the cutting knife from continueing into your frame. And always put your ruler, or what ever you using as a guide when you do the cutting , on the side that will be the frame, so if you slip you will only cut the middle peice that you wont be using.
Hope that made sence.....sorry if measurements sound weird ....im from NZ ....so don't think you guys use cm and stuff
I can get a pic of one of mine framed up if you want to see what im on about......or I can try explane better if you need me to