Here's a link to a gestational chart: http://www.cattletoday.com/gestation.shtml
. For most breeds, it's 283 days, but a cow can calve as much as 2 weeks before or after her due date and still be in the "normal" range.
If your heifer is in a large pasture, I would say yes, bring her into a smaller, secure enclosure where you can easily keep an eye on her. It sure beats wandering around a 10-acre pasture in the dark looking for a cow in labor. Make sure she has shelter, water, and hay.
When I turn the bull in with cows or heifers, I watch for activity and mark it on the calendar. Then I make a note about 19-20 days from that date so that I can watch the cow/heifer for any signs of heat; this is not a sure thing, but helpful.
If the bull is with cows year-round, you may have cows calving at all times of the year -- deep winter, hot summer, etc. In your location, I would think you'd want your cows to calve in the winter. Summer calving is VERY stressful; people lose cows and calves due to the heat. Cold weather calving is easier on cows and calves than hot weather. Also, weaning several calves together is easier on everyone than weaning one at a time throughout the year.
The heifer that aborted could be pregnant or may not be pregnant. Your vet can do a pregnancy check or draw blood for Biopryn testing and then you'd know for sure.