How to cause the value in the denominator change with changing row

I'd like the value of the denominator to change as I key in each day sales automatically. =SUM(C31/6*23) in this example 6 is the day of the month and 23 is the number of business days for the month. Is there a way to define 23 as a formula to count the number of business days and return the value 23? Is there a way for the value in the position 6 is in to change as I key in daily sales? I've attached the spreadsheet for help. thank you

STORE MONTHLY TOTAL
MARCH ABC ABC
DATE DAILY MTD DAILY MTD
1 12,747.71 12,747.71 3,266.29 3,266.29
2 9,319.74 22,067.45 2,871.36 6,137.65
3 16,619.81 38,687.26 5,073.21 11,210.86
6 20,166.18 58,853.44 5,999.47 17,210.33
7 14,562.88 73,416.32 4,376.16 21,586.49
73,416.32 73,416.32 21,586.49 21,586.49
8 29,382.66 102,798.98 6,852.76 28,439.25
9 0.00 102,798.98 0.00 28,439.25
10 0.00 102,798.98 0.00 28,439.25
13 0.00 102,798.98 0.00 28,439.25
14 0.00 102,798.98 0.00 28,439.25
29,382.66 102,798.98 6,852.76 28,439.25
15 0.00 102,798.98 0.00 28,439.25
16 0.00 102,798.98 0.00 28,439.25
17 0.00 102,798.98 0.00 28,439.25
20 0.00 102,798.98 0.00 28,439.25
21 0.00 102,798.98 0.00 28,439.25
0.00 102,798.98 0.00 28,439.25
22 0.00 102,798.98 0.00 28,439.25
23 0.00 102,798.98 0.00 28,439.25
24 0.00 102,798.98 0.00 28,439.25
27 0.00 102,798.98 0.00 28,439.25
28 0.00 102,798.98 0.00 28,439.25
0.00 102,798.98 0.00 28,439.25
29 0.00 102,798.98 0.00 28,439.25
30 0.00 102,798.98 0.00 28,439.25
31 0.00 102,798.98 0.00 28,439.25
TOTAL 102,798.98 28,439.25

"SPEED" 394,062.76 109,017.13
GOALS ABC GS$: 331,793 ABC GP$: 106,279
Daily GP$ avg. required to meet
monthly GP$ goal 4,620.83

Daily GP$ avg. to date: 4,739.88
Last year 3/05 Sales$ 293,622 3/05 GP$ 95,566