Rolling Interval Target Needed to Achieve Goal

Hoping someone with a mind for math and DAX can help!

Scenario:
A call center has a service level goal of answering 80% of calls offered within 60 seconds each half hour interval.

By the end of the 9:00 AM interval, only 369/570 total calls offered have been answered at service level goal (64.7%).

There are 1,854 remaining calls forecast for the remaining intervals for the day.

The 570 calls offered plus the 1,854 remaining calls forecast equals an estimated total of 2,424 calls for the day.

Problem:
Given the ratio of calls answered at goal by the end of the 9:30 AM interval (64.7%), how many calls during each of the remaining intervals for the day need to be answered at goal?

My approach so far:

Measure = (((TotalOffered + Forecast Remaining)*.8) - TotalAnswered) / Forecast Remaining

Problem-solving reasoning:
2424 max possible calls offered

2424 x .80 = 1939 total calls answered in service level needed

1854 forecast calls remain

369 calls answered in service level

1939 total calls answered in service level needed - 369 calls answered in service level = 1570

1570 / 1854 forecasted calls remaining = 84.7% of calls answered in service level needed for rest of day

Where I'm stuck:
The measure is including the intervals that have passed already and I don't think it should since the remaining intervals are the only chance to move the average towards the goal of 1939/2424 total calls answered.