I have just installed a five raised beds, each being 3.6m x 1.29m x .2m (12foot 4ft x 8 inches).
They were built elsewhere in the garden and then put in situ, so they sat on the existing grass.
I then removed the grass double dug the soil and buried the grass under the bottom spit. I also added four wheel barrow loads of well rotted horse manure to each bed.
This raised the beds by about 3 inches (Would probably settle down to about 2 inches.
I then added about 40 to 50 full wheelbarrow loads of good topsoil (Neighbour building a large shed) to each bed and this raised them up another 4 inches to about 2 inches from the top but they will settle so realistically half full.
But and this is the important bit, raised beds grew out of the medieval practice of "No dig" beds that were often surrounded by small wattle fences and the allotment practice of deep dug beds that raised up the soil, now formalised by adding the wood etc.
The soil does not need to be up to the top of the wood. It will raise up over time as the ground is worked and compost added each time a plant is planted and home made compost is dug in each year.
There is no need to add any compost to a new raised bed as long as you dig the underlying soil and then keep off (Apart from a light firm of the Brassica bed), the boards will act as a windbreak making a micro climate in each bed.
And it will happen with time like most gardening techniques, and your beds will be full up in a couple of years for free :happy:
Bob