Short answer: You don’t need to sit “before” sunset. Evening (Devasi) Pratikraman is to be done at sunset or within one muhurt (about 48 minutes) after sunset.

Practical guidance:

• Best: begin right at sunset or within 48 minutes after.
• Many people settle a few minutes early (5–10 min before sunset) just to get seated and compose the mind, so the main vidhi starts at sandhyā (sunset) itself.

Tradition notes:

• Shvetāmbara: evening Pratikraman is after sunset; aim to complete it within one muhurt.
• Digambara: also after sunset; some sanghs perform it a bit later in the evening but still within the early night period.

For a simple reference on timings: and a quick Q&A: