I'm one of those writers who uses plot boards and I'm currently in the process of creating a new one for my current book. I experimented with a scene board and I think I've ended up with way too many scenes for the length of the book. I am going to have to make some decisions about setting as well and that will change the beginning of the book, too. Oh the woes of revision!
I have gone back to what I consider the standard board for a 400 page manuscript. The one I usually create for a book and this board always gives me a good idea of where I am in the story and what needs to come next. It is a basic twenty block board. I did 4 rows with 5 blocks, also left a top space for a prologue or general notes. Each block is twenty pages. The first 5 blocks equal 100 pages. By the end of the fifth block, the beginning of the book should shift into the middle. The fifth block should contain the first turning point or "surprise" as Evan Marshall calls it.
Blocks 6 through 15 are the middle. I consider 6-10 the first half of the middle and 10-15 the last half. Usually if a middle is going to sag, it will be between blocks 8-12. So I think a little surprise around block 10 is a good idea. By the end of block 15, a major turning point should shift the book into the ending.
Blocks 16 through 20 make up the conclusion. This means tying up loose ends and subplots. By block 18 the story crisis or black moment should be set into motion, followed by the "turnaround" in the plot when the lead characters face the opposition, fueled on by a new goal/change. Marshall refers to this as the Saving Act. By block 20, the lead character has achieved success. In a romance novel, this block contains the HEA ending.