Most CO2 systems do not allow you to effectively reuse the remaining gas in a cartridge. They're a use once and discard item. There are systems that allow you to meter the flow and stop half way, but they tend to slowly leak over time, so you cannot rely on that half empty cartridge you're saving.

CO2 is fine for small tires such as 700x25c where one cartridge can fill a tire to decent pressure. But on larger tires they won't and you have three choices.

1- use the CO2 for the immediate emergency, and ride home on the partly filled tire

2- use the CO2, and top off with a pump. This is a poor approach since the CO2 did the easy part, and you're left trying to do the hardest part of the job, which if you could, you might as well have skipped on the CO2 altogether.

3- pre-fill the tire part way with your pump, and use the CO2 to finish the job. The right amount of pre-fill depends on the size of the tire, cartridge and desired final pressure. You can estimate the amount by using one cartridge to fill an empty tire, and noting the pressure achieved. Subtract this form ideal pressure to get the amount of pre-fill pressure needed.

IMO, pre-fill and top off is the smartest way to use CO2. It allows you to confirm seating and centering before using the one-shot unit, and gets you to full riding pressure without serious pumping effort.