We've had this feature for 10 years in our previous shopping cart, which was a Projected Sales metric for the current month. It worked very simply, if you had sold $9,000 worth and it was March 7th, then knowing that there are 31 days in March, the projected sales are $39,857.
This metric has become our go-to way for knowing how well it's going (or not). In the beginning of the month, the numbers fluctuate, but as the month progresses, you start to trust this number. In fact, in the last many years, I've never really looked at the month-to-date sales, the numbers don't really mean anything compared to the Projected Sales metric.
X-Cart should also have a Projected Sales metric. It's super easy to calculate, just divide the number of days in the month by the current day of the month and multiply by the month-to-date sales amount.