Hey Paul. Fundamentally I don't think it is critical; assuming you have enough to do the knot on either side of the marks, only the spacing between the marks is critical. However the line in your photo looks like polypropylene to me; Dyneema's a bit pricier so ideally you don't want to waste too much of it. If you want just enough to be able to tie the knot, then the original line length is more important.
s/v Laelia - 1978 Pearson 365 ketch
s/v Essorant - 1972 Catalina 27