My cabinet is 33" wide and I have 4 dividers, or 5 cubbies. I love the spacing. I can fit all my cookie sheets/jelly roll pans (about 6) in one cubby. Cutting boards are in another and the rest are used for platters. It works out great. I would think for 24" cabinet, I might only use 3 of the dividers because that would make them each about 6-8" depending on the thickness of the actual divider which is the same width I have for items. Hope that helps!

The inside of my above the oven cabinet is about 29'' wide but it is also about 28'' high and 24'' deep. The 4 tray sections have 3'' spaces between them. That spacing works great for my trays. As you can see the rest of the cabinet is available for other storage. Sorry forgot to resize the picture.
{{!gwi}}

Have you measured the things you want to put up there? I wouldn't plan on too many items per divider because you don't want to pull them all down on your head, but of course muffin tins are wider than cookie sheets and so forth. You could duct tape to hold the dividers in place and experiment with the actual things you're planning to store, sliding them in and out, so you can maximize the storage.

Here's mine...if you haven't already seen it...

They're perfect for just a few items in each "slot" so no searching for what I want...

Mine are above 30" ovens. There are five dividers, so six slots, about 5-1/2 inches between each one. If I were you I would make a few narrower slots for platters, trays, cooling racks and cookie sheets, and a few wider for bigger things. Eighteen inches is pretty tall so you may want to have a horizontal shelf or two like buehl. Look at what you want to store up there and figure out how much space your items need. You may be best off with an arrangement like badgergal's.

Here is mine. You can see that the cabinet is used for other storage below. The actual tray section is about 22" tall.