You can use the recommandations for OECD 407: https://www.oecd.org/env/test-no-407-repeated-dose-28-day-oral-toxicity-study-in-rodents-9789264070684-en.htm (18. Dosage)

Namely, if you have no acute effect at 1000 mg/kg bw, you use it as point of departure. Otherwise, you have to determine the no acute effect dose you found in your acute study.

you can use Ld50 200mg/kg

How?