Chinese Cabbage (Napa) (cool-season heading brassica) is classed semi hardy in the cited extension guidance. The timing rules below are relative to your local freeze dates — enter a location to turn them into calendar dates.
The rules this site applies
Method
Frost-relative window
Rule & reconciliation
Start seeds indoors
8–6 weeks before the median last spring freeze
Arkansas Extension requires transplants no older than "four to five weeks" at setting out, so indoor sowing is counted back 28–35 days from the cited transplant window, giving last spring frost minus 56 to minus 42 days.
Transplant outdoors
3–2 weeks before the median last spring freeze
Arkansas Extension states "Transplant two to three weeks before the last frost-free date", giving last spring frost minus 21 to minus 14 days; Clemson's warning that spring-frosted seedlings bolt is a caution on the same window, not a competing interval.
Variety assumption: Offsets assume spring-set transplants of the 45–72 day Napa and Michihili types tabulated by Arkansas Extension; no fall window is encoded because every fall rule found for napa is fixed-calendar ("early to midsummer", "July") rather than frost-anchored.
"Chinese cabbage is best sown in late summer and autumn rather than spring. If seed is sown in spring, young plants may bolt if they are frosted or exposed to long, cold nights."