Maybe you should change the title of the question to
"Dividing shares to part time founder in start up"
because it seems like you're focusing on the part time status more than the fact that he's a graphic designer.
Seems like the easiest way to do this, is to just calculate the market rates of everyone's contribution.
Let's say that you're hourly is $100/hour (I'm making out numbers), the biz/product guy is $80/hour and the designer $60/hour. Also, let's assume that you and the biz guy are going to put 60 hours a week and the designer is going to put 15 hours a week.
So the total contribution of everyone is
You should get 51%
The biz guy should get 41%
And the designer should get 8%
Which "feels" right to me.
By the way, make sure that you do the paperwork so that everyones shares are tied to how long they stay in the company, and that if someone leaves, the other two have the right to buy them out.