You don't need half-unit markings - just do it by eye. I use 3/10 ml (30 unit max) syringes. They are marked only on exact unit measures, but the marks are far enough apart that I can estimate anywhere inbetween and sometimes will especially when doing corrections. Besides exact unit measure I will sometimes go for: "slightly over", "one third", "one half", "two thirds", "slightly under".
This is one reason I prefer syringes over pens. (And they are half inch).