No. That is not Navkarsi.

Navkarsi means:

• At night: no food (and usually no water) after sunset.
• Next morning: after sunrise you wait about 48 minutes.
• Only then you take the first food or drink of the day.

If you eat or drink anything at night after sunset, or in the morning before that Navkarsi time, then that day you have not done Navkarsi. You can start Navkarsi properly from the next day by:

1. Doing chauvihar (no food, no water) after sunset.
2. Next morning, waiting 48 minutes after sunrise.
3. Then taking your first food/drink.

So eating till 12 pm and then saying “I did Navkarsi” is not correct according to Jain discipline.

You can read a simple explanation of Navkarsi here