If you have a column of dates listed in column A for example, and the spending per day listed in column B, and the monthly budget in a cell, say D1, for example during the month of March then you could set up this formula in another cell to determine the target spending per day in the future:
Remaining budget would be
= D1 - SUM(B1:B31)
Remaininng days in the month would be
= A31-TODAY()
So then the answer would be
=(D1 - SUM(B1:B31))/(A31-TODAY())