as long as the disk is less than 80%full it should never fragment much.
above 85% it becomes noticeable and above 90/ it is clear, beyond 95% it is bad. there is a slidign scale here, if the disk is 10Mb that might be less, if it is 250Gb ... the files would rarely be 1% so 95%should work ok.
it is the amount of room left to store the file in hat matters.
if there is enough free so that the file can be written in full everything is fine.
but if only scattered places are available and the file must be split disk starts fragmenting.