This tough subject came up again the other day. It’s really hard. As Graham Lister once explained, the issue of Parallax also needs to be taken into account. There’s a really good write up of this here:

http://www.tallgums.com.au/fixtures-line-calls.htm

Oh – and for your viewing pleasure, even those that are paid to get this right often get it wrong. Check out this YouTube here:

https://youtu.be/IsyXG8Xhcsg