I think Carmarthen county council & lots of others have something on their planning website that will calculate the plotted area you chose to enclose .
Due to the memory problems I now seem to have I can't say much more other than I used it several years ago to check the area on a Google Earth map in a 10 mtr view that I put the boundary points on .
My learned Solicitor who has supervised me purchasing more land once pointed out to me that a boundary can be anywhere along the width of a pencil line on the map . So be aware that it might be any where in the metre width of the line shown on the map when you are looking for them on the ground .
He also advised me to put in plenty of the metre long concrete boundary posts that carry the years date cast into them , as well as getting someone with one of the latest satnav devices that can give sat nav coordinates down to less than a half a metre reference point to actually give you lots of co-ords as you walk the lands boundaries .