Luckily you are not required to use a ruler or be exact- so it just has to be in the range. I would (in pencil or erasable color) divide the line into 4 sections as guide points then determine where each of the sections would go. EX: If it was 4/10 I know it is closer to the 1/2 mark then the 1/4 mark and place the dot.
If you need to, find where the marks would be before simplified-- 3/10's half way guide point isn't 1/2 its 5/10.