You can buy u100 syringes that have 1/2 unit markings, so that would remove some guesswork. Basically, just halfway between the 1/2 hash and the next line. Still a bit of guesswork. There are lots of people who do it though. I used u40's with only whole unit markings, so anytime I shot a fractional dose, it was eyeballed.
It amazes me that some people move up or down in .1 increments. I couldn't have done that.
Hopefully someone with "microdosing" experience will be along to help you out,
Carl in SC