Very pleased to hear Tinkerbell is coming home today, Fiona! OK! So...
Re the Sheba - I don't know the carb content v what the vet has been giving her, but you can check this using a calculator that
@Elizabeth and Bertie can give you. The Hills does have sugars so that might be contributing to keeping her bg high, so in theory the Sheba is probably a better bet, but until you make the change we simply don't know.
It's the insulin dose that is the big question mark at the moment. That 5u or 6u looks very high for a recently diagnosed cat, but we don't know what else is at play that might be keeping the bg high and requiring that dose. Also, when you are changing to a lower-carb food, that might lower bg and therefore reduce the need for that dose. It's hard to directly contradict your vet, but most cats are started on a much lower dose - 1u twice a day - and see how they get on with that, increasing only slowly until a dose is reached that is keeping the bg in better levels for most of the day. A bg of 30 at the vet's is not good and does beg the question of whether it is too high, causing a reaction that Kris explained very well a few days ago. So dose is something you need to think very carefully about, discuss with the vet and bear in mind what you've learned here. Personally I would certainly not give 5u. I might drop to 2u and see how that goes - it might still be too much so you might decide to start from scratch at 1u.
As you know, the main thing here is to know how the insulin is working at what dose, with the aim of making it as effective as possible (ie as much as she needs to keep her bg within acceptable levels) and keeping her safe (ie not too much which could risk hypo). Unless you are very lucky, this is not a quick process. It is a fine balancing act and you need to build up more data (bg tests) to gain an awareness of how Tinkerbell *might* respond to a particular dose when she is back home.
Basically, you need to test BEFORE EVERY SHOT and also, in these early stages, at possibly significant points during the cycle as well. So that might mean two hours after the shot (+2), followed by a +4 and so on. With Caninsulin we'd expect a good drop early in the cycle so that's when you need to test to make sure she isn't dropping too much/fast. Later in the cycle we'd expect to see a steady rise. BUT it's important to point out that every cat is different and what we *expect* to see isn't necessarily a certainty.
You will need to log these numbers on a spreadsheet and link it to your signature here so that if you post with any questions, we can see at a glance what dose you've given and how Tinkerbell is responding to it. There are technical types here who can help you set up the spreadsheet quickly, and then it will be easy for you to maintain.
Sorry this is long but I've tried to cover all bases. It may seem a bit overwhelming at first but you'll soon get the hang of it. As I said recently, knowledge is power. There may be more questions so keep asking, we will do our best to help.
Good luck and above all enjoy having Tinkerbell back home!