I prefer about 90. But any time between 80 and 100 is fine. 70 to 80 and 100 to 110 days is possible but harder and the later or early lambing ewes can be less accurate.
Twins show up at 45-50days so two cycles forward from there gives you 80/85 days. Pregnant or not pregnant shows up about 35 days. Some ewes show earlier and some ewes show later. Just scanned ours an 200% result well chuffed