This makes it very difficult
Kili, seconding what Suzanne says...
Small doses of U40 insulin can be
much easier to measure in U100 syringes.
I'm pretty sure that someone must have explained this to you already, but just in case it hasn't been explained, U40 insulins (Prozinc and Caninsulin) have 40 units of insulin per ml. U100 insulins have 100 units per ml. So there are 2.5 times the number of units in U100 insulin.
...People measuring small doses of U40 insulin can use U100 syringes to great advantage, because there are more 'usable lines' on the syringe barrel. So, for example, one unit of U40 insulin will measure to the 2.5 unit mark on a U100 syringe.
Quite a few folks using U40 insulin do use U100 syringes, along with the conversion chart that we have here.
Something to be aware of though is that the lines on some U100 syringes can be a bit inaccurate for the tiny doses that we use in cats. This is because they're made for use with human insulins, and humans generally need much larger doses.
The most commonly used U100 syringes among UK folks are the BD U100 0.3ml '+ demi' syringes ('+ demi' means they have half unit markings). These are the most widely available U100's in the UK. ...But many who use them recommend that they are used with digital callipers in order to accurately measure the doses.
...I don't use callipers. My own method is much more low tech. I keep a well marked syringe as a 'template' to compare other syringes to. ...So, I draw my template syringe plunger to the dose I want. Then I draw insulin into a fresh syringe, and compare the two syringes side by side, and adjust so that they are the same. ...My close vision isn't great. So I keep a craft magnifying glass on my kitchen worktop to use to measure the doses accurately. ...This works just fine 'for us'. But some others might say it would be easier to use callipers, haha!
There are some veterinary U100s available, made by Sol-Vet. These are less widely available. But folks who use them generally report that they have more accurate markings than the BD U100s.
Eliz