Scarlets Cut Flower Patch 2022/23/24

So now I am thinking perhaps Im not as old as I look and it is possible to do 6 new beds for dahlias if I can get my cuttings to take. I am thinking card/compost and plabt cuttings straight in.
Compost cost for 6 beds?
Anyone know how to so the maths?
6x30FT?
How thick would i need to go.

So six 6ft x 30ft beds? Each bed would need 425L per inch, so 2550L in total. 

As for the depth, if you are using cardboard then the compost is mostly for nutrition rather than mulch, so I think an inch or two would be fine.