I'm not sure having boards in 10cm increments is that cool. It can work, but it depends on board size and what conditions you want covered. It can create a bit of a gap between 65/101lit and 54/74lit.
I use a 75 wide slalom with a 7.3 and then jump down to 59 wide Kombat (with a 6.0). The overlap point is around 18 knots (solid) to 21+(holey). It was my choice to minimize quiver and it works, but sometimes I wish for a 100 lit board for 6-7m sail (I'm 86kg/190lbs).
I guess we all search for a perfect quiver, and it never ends ;-)