I debunked a demonstrably incorrect argument. Clearly I was out of line. :-/
Nope, you are not out of line... we need people to challenge the findings... all of them. Your math appears to be correct... the percentages of old to new carbon-14 are probably correct given your assumptions (We have no knowledge about the percentage of carbon extant in either the linen or the bioplastic residues - for example, linen may be only 10% carbon and the bio-plastic may by 90%, or vice verse, which would skew the math completely).
Now we need to go to the source and examine the actual observed percentages of original linen and its C-14 to the percentages of ancient to modern bioplastic micro-organism residue and see if they match your math.
I recall reading a peer reviewed article in which that exact investigation was done... and the report seemed to show that 2/3rds of the sample thread WAS bioplastic residue. I will have to go looking for that report. I believe it is on WWW.SHROUD.COM.