OK - I found the email from the late Howard Madaus that analyses the Watkins & Ficklin invoices. I will post those invoices first and then Howie's analysis. I will put how each figure of cloth actually measures out too.
Oct. 15, 1861 - Watkins & Ficklin
1 yds. blue silk 16' 1.12 $18.28 (1 yard width/16 feet long bolt)
2 yds blue silk 93 1.50 $139.50 (2 yard widht/93 feet long bolt)
1 yds white silk 83 1.50 $13.13 (1 yard width/83 feet long bolt)
1 yds white silk 23' 1.75 $40.69 (1 yard widht/23 feet long bolt)
1 yds white silk 5 3/8 2.25 $12.09 (1 yard width/5 3/8 feet long bolt)
1 yds red silk 8 3.50 $28.00 (1 yard width/8 feet long bolt)
Oct. 18, 1861
2 yds rich pink silk 111' $1.30 $166.88 (2 yards width, 111 feet long bolt)
Oct. 28, 1861
4 yds ribbon 3/9 $2.50 (4 yards length/ 3/9 inches wide)
7 yds ribbon 3/9 $4.38 (7 yards length/ 3/9 inches wide)
Oct. 29, 1861
18 (2) blue silk 1.622 $30.06 (18 feet long/ 2 yards width bolt)
Nov. 4, 1861
for 69 yards blue silk 1.50 $103.50 (no widths mentioned)
24 yds blue silk 1.00 $24.75 (no widths mentioned)
Each flag measures out in four red/pink triangles that are 36 inches wide base by 18 inches to the peak. From one yard of 1 yard wide silk four sections could be made (one flag); one yard of 2 yards wide silk makes two flags.
Thus for the Oct. 15, 1861 invoice we can get 8 battle flags; from the October 18 invoice we can get 74 flags. Thus the two invoices make for 82 flags. This is based on each flag being 48 inches square which surviving examples seem to be.
So if Selph's recollections of 120 flags, assuming they are right, we are short some flags. Howie surmised that the red silk was for the three Cary girl's battle flags (Van Dorn, Joe Johnston, Beauregard and some others) while the rich pink silk was for the flags as issued to the army. This might not hold up of course.
Alan's finds have invoices with more red and pink silk listed:
Oct. 22, 1861 - 2 yards red silk (width unknown)
Oct. 26, 1861 - 7 yards pink silk (width unknown)
If the above bolts were 1 yard wide we can get 8 - maybe 9 - flags. If the bolt was 2 yards wide we can get double that. So let's say 18 more flags from these invoices. Added to the Watkins & Ficklin invoices we have a total of perhaps 100 flags tops. So if Selph is correct with 120 flags then we have to find more invoices for red/pink silk to make them.
The ribbon, by the way, is for the flag's fimbration.
We obviously have huge amounts of white and blue silks and it would appear that both suppliers that Alan posted here that sold yellow silk was for the external borders for those flags that used them. For a breakdown of the three models of ANV silk flags we have:
1) Two variants of one model - white silk hoist sleeve with gold fringe; white silk hoist sleeve with white silk borders. The gold stars are painted on the blue crosses.
2) Blue hoist edge with yellow silk borders. The stars are white silk sewn to the blue crosses.
So these will account for the blue and yellow silk and some of the white as well. But it still leaves a lot of extra silk for these flags that had to have been used for something else.
We do know that the first 34 First National battle flags for the Army of the Potomac (CSA) were made in Richmond by George Ruskell but these were made of wool bunting taken from the captured Gosport Navy Yard near Norfolk. So these cannot account for the extra silk.
Anyway - hope this has been helpful. Thanks again Alan.