Using 320 as an early healthy date and 342 as an average healthy date:
First exposed is April 29th, 2012
320 days gestation = March 15, 2013
342 days gestation = April 6, 2013
Last exposed May 30, 2012
320 days gestation = April 15, 2013
342 days gestation = May 7, 2013
If she is housed outside, some literature discusses those mares will be on the longer end of average (340-370) My mares are outside until a couple weeks before their expected foaling date when they come in at night only. Using that method, they have tended to be 340-350. After reading that study, I'm bringing my broodmares under the lights & heat earlier than normal. This will hopefully get them to foal a few days sooner than their personal averages.
Looks like you will need to watch your mare starting now and really be vigilant with watching her beginning in March. Being that she is maiden, you will have your work cut out for you. This is the problem with pasture breeding - you have almost zero ability to calculate when the mare will need certain health care like vaccinations, worming, pregnancy scans, etc. Without ultrasounds at certain intervals, you cannot check the fluid or check for placentitis, etc. There is nothing to do at this point other than provide good nutrition and update her vaccines and deworm as necessary. Get ready to wait, and wait, and wait some more.
I would suggest starting a chart. Take her vitals 2x/day if you can. A caliper is best, but if you have a soft measuring tape, measure her udder size and length of her vulva daily. Grade her ability to resist tail lifting as well as the laxity of muscles in her hind end. Use all this data to establish a baseline, then you will have an idea of when she is nearer to foaling.