The general rule in that scenario, giving a late shot, is that now becomes your new shot time. Next shot should be given in 12 hours. With Lantus insulin, shot times can be moved 30 minutes once a day or in 15 minute increments twice a day.
You can also skip the shot entirely. Pick back up in the morning, knowing the number is going to be higher.
With experience and enough test data, when you know better how your cat would react to the insulin, when the onset is, when the nadir is, how long the duration is. In that case, you could do a reduced shot, like half. Not recommended for someone in your situation, without much test data.
late shot acts like a dose reduction
early shot acts like a dose increase
One skipped shot is only a tiny setback. "Better too high for a day, than too low for a moment. "
This is also when you want to teach another person in the family to test and shoot.