The Forecast Spend value within a budget uses the average daily spend over the last 7 days as the per daily estimate for the remaining days in the period to generate a projected spend figure. It then adds the spend to date and this projected spend to create the Forecast Spend.
For those who love math, the actual calculation is:
(spent to date - today's spend) + (number of days left (inclusive of today) * (last 7 days spend / 7)
Example based on a 30-day period:
Spend for the first 15 days of the period = $20,000
The average daily spend for the last 7 days = $1,500
15 days remaining in the period = 15 days x $1,500 = $22,500
Forecast = spend to date $20,000 + the project spend of $22,500 = $42,500
This will be updated hourly.
You can see the Forecast Spend in multiple locations in Adpulse:
Column on budget page