Around 1970 at the campus of UCLA, a young Dick Vermeil -- then the offensive coordinator -- worked with then-head coach Tommy Prothro on devising a chart to determine when to attempt a two-point conversion. Every now and then when you're watching a game on TV, the announcers might refer to "the chart" which most head coaches have in their back pocket in case they need help figuring out when they should "go for two."
Of course, coaches also have to determine when it's the best time to go for the conversion instead of the extra point. For example, let's say you're coaching and your team is losing 22-0 but they score a touchdown, cutting your deficit to 16. Do you go for two to make it a 14-point game? It's not an easy choice -- especially if it's in the first half or early in the second half. Truth be told, it's not always easy to know when to attempt a two-point conversion.
Just in case you ever need it, here it is -- a version of Prothro and Vermeil's two-point chart.