In Jain practice, a pachkhan should be taken before you begin the fast, not after eating has already stopped.

Simple rule:

• Take the pachkhan at the time you resolve the fast.
• Ideally, it should be taken before the prescribed fasting time begins for that day.
• If the fast has already started, then taking pachkhan later generally does not make it a proper vow from the beginning.

Different Jain traditions and local practices may have slight differences in the exact vidhi, especially for aatham, chaudas, upvaas, ayambil, and other vrats. For complete correctness, the pachkhan should be taken in a proper state of mindfulness, before commencing the fast.

If you want, I can also explain the difference between pachkhan for upvaas, ekasana, besana, and ayambil.