A closer look at fine doses--see the syringe at top.
Even when you can't dilute insulin, it's possible, with a lot of patience and a good magnifier, to dose insulin in fine gradations. Since a small dog or cat may weigh 1/10 what an average human does, it shouldn't be surprising that a 1 unit change in a human is sometimes like a 1/10 unit change in a small animal.
With U40 insulin[]
With U40 insulin, you can dose insulin to a precision of 0.2 units by using a U100, half-unit marked syringe, and converting.
With U100 insulin[]
But with U100 insulin, all you can do is use patience and good eyesight to "eyeball" fine doses. Consistency is more important than accuracy -- if you can give the same amount reliably tomorrow, you have succeeded.
It is indispensable to use half-unit marked U100 syringes (3/10 cc with half-scale marks also referred to as low-dose). [1]
Pictures method[]
The Fine Dose Pictorial Guide shows one numbering method.