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 insulinEdit
With U100 insulinEdit
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). 
The Fine Dose Pictorial Guide shows one numbering method.