Edit: beaten to it! At least you have the formula now.
Darrell, using
this link we know that 70000000 cubic mm is equivalent to the 70L tank you require.
Dividing that number by 685 then by 600 I'm left with 170mm as the required depth.
I'd go slightly deeper as these are outside dimensions and you will lose a bit to the tank walls, baffles, internal pump (if applicable), pickup, etc.
Whatever dimensions you choose, multiply the 3 together (in mm) and divide by 1 million to get the litres.
Cheers,
Paul