Hi guys. Hope somebody can help me.

We are currently sorting out out back garden for lawn seed. Have dug it over
to a depth of about 6-8 inches. It formerly had a layer of shingle on top of
it which has been seived and removed.
We now need to put down some top soil but are not sure how much to get and
how thick it should be. The soil that has been dug over is a bit sandy but
apparently that won't cause a problem.
The garden is about 40 square meters.

Any help much appreciated.

Pete