Mix for boxes

I started growing vegetables a couple of years ago; and as is my usual way, I decided that I would need a reference book. I did a Google search for "Square foot gardening" not surewhere I'd heard of that, but out came "`Mel Bartholomew's Sq Ft Garden handbook".
I was probably just looking for raised beds or deep beds, but liked the look of Mel's method.
If you are not familiar with it; a brief description is necessary.

Basically 6" deep timber boxes, 4' X multiples of 2'. Mine are 4 X2, 4 X 6, and now 4 X 10. (the 4 X 2's are made for plastic mini-tunnels and work very well.

The basis of the system is the compost mix, which is rather expensive, 1 part coarse vermiculite : 1 part Sphagnum moss : 1 part mixed general purpose compost. This is a bit pricey for large beds.

Planting spacings are given, and advice on other important aspects like watering etc.

I'd now like to make my own "mixture" and have in mind using Seived topsoil; horse manure (wood chip based) : My own Dry method compost.

My new beds will be 8" deep, to give a bit more scope for crops like carrots, and to give cabbage and cauli's etc a bit more root stability.

I use a membraine on the bottom of the boxes, and don't overdig the area under the boxes, in fact latest boxes will be on a hardcore area with bark paths around each edge.

Any suggestions, for a good nutritious mixture, and any other things to consider?

Also; any recommendations for maintaining nutrients in Mel's compost year on year; and when is it time to start with a new mix; must run out of goodness at some point?

regards
alan

Hi Yorkie1; point taken on the nutrients aspect, I'll get the books out.
My existing beds are only 6" and have a black fabric membraine over what was lawn. I've moved them around a bit as I've been allowed to use sunnier areas of the garden! Not dug under them at all, in fact not looking for roots to go any deeper. I do get a bit concerned about stability of cabbages, calabrese and cauli's, but I'm doing ok with them considering I'm learning as I go!
Courgettes were very prolific in 6" of compost last year, and never looked like falling over.
My concern was that the compost was very "light"not very cohesive, so I thought a homemade mix with some decent topsoil and composted vegetable matter would maybe stick together a bit better and provide better anchorage.
regards, alan