I filled mine to within an inch from the top (just did it yesterday) and used the traditional Mel's Mix of 1 part compost, 1 part peat moss and 1 part vermiculite. I don't believe it will settle but have some extra stored in bags if need be. Maybe some more experienced gardeners can suggest something more definitive. Good luck!

Here is a link that might be useful: Sinfonian's Gardening adventure

The idea with lasagna beds is that you will always be adding to it, if you don't they will shrink to about half of what you started with, so just keep adding what ever ingredients you are using.
Alberta

You have 18" of planting media. It will definitely shrink (massively if you did it lasagna style), but for a season it is more than enough. Plant and enjoy!

Thanks everybody. :) I put in more leaf mulch/poo and straw, then the bagged manure and compost. Today I planted snap peas, lettuce, spring onions, spinach, and carrots using leftover seeds because MY DRATTED GURNEY"S ORDER IS STILL NOT HERE!!!! (can you say unhappy?) Also planted the broccoli in separate bed and containers, and about a dozen raspberry plants in their own bed. Also put some Daikon radish seeds around the broccoli. YUM.

After that I used landscape rocks and built a "stonehenge" around this sad rose bush of mine, and did a lasagna thing inside, to see if I can help the bush look better. (CAN YOU SAY STIFF AND SORE??? It is now almost midnight here; I've been in bed since 9:30 but despite Advil and hot rice bags, I can't go to sleep b/c I hurt so much so here I am writing... ) Also think I got a sunburn, LOL.

In little peat plugs inside, I have beets, thyme, carrots (can you say DESPERATE TO PLANT SOMETHING?), cucumbers.

Guess wht my next bed is going to be? DW brought home a deeper wheelbarrow, so I get to use the old one for a plant bed.

L