To mark my cuts, I placed the Homasote on top of the drawer upside down, then traced the edge of the drawer.
After flipping the sheet over, I clamped a straight edge along the marked line, then started the cut.
Because I was using a 4' level for the straight edge, I extended the knife blade far enough that the handle would clear the body of the level - this is one advantage of using an extendable blade over the traditional utility knives.