Could you explain why you think the surplus cash is around $70m?...

Could you explain why you think the surplus cash is around $70m? I know you mentioned before that you're looking at the change in working capital of $34m from contract liabilities and subtract them from $102m cash, but why would you use change in working capital to determine surplus cash instead from the balance sheet of $47.7m? (from "Contract and other liabilities")

I calculate the surplus cash to be $58.4m, this is the difference between the Current Asset and Current Liabilities. I know MarsC believes it is $55.2m, by subcontracting the $47.7m in "Contract and other liabilities" from the Balance Sheet, however my reasoning is based on the fact that CA minus CL should be 0, therefore each dollar of short-term liability can be covered by each dollar from short-term assets, following on from this, any excess short-term assets can be considered "surplus".

In fact, I also add the $886k from "investment in listed securities" (from Non-Current Assets) as surplus cash too, so surplus cash is $58.4m

Any thoughts?