What insulin are you using? If it is Lantus, please go post there, as someone very experienced should be available. My cat hasn't had many complications (yet).
As the insulin takes effect, she is likely to become more hungry and eat the remainder, but you can't bet on that.
A half dose is what my vet had me do pre-dental when my cat was fasting, so that might be reasonable.
Has she had problems with ketones ever? If so, you might do the full dose and then do serial glucose tests to monitor her.