The amount of receivables measured in dollars is meaningless and can potentially mean different things to different people. The measure we have used for many years is the percentage of one month's average business that is in receivables. So, for example, if one has $10K in receivables, that amount is quite different for the company that averages $7500 a month in sales (133%) when compared to the company that averages $75,000 a month in sales (13.3%).
In our accounting system, any job which has been contracted for and has not been paid for in full becomes part of our receivables. Thus the $1000 in unpaid balances of completed work sitting on my pickup table is part of receivables, as is the job I booked yesterday for which I accepted a 50% deposit but haven't yet even begun to fabricate.