Yep those contractions are the best to look out for. Ignore the time between them as it can be erratic, for me they were always anywhere between 5-10mins apart, alternating between strong and weaker contractions (weak enough that I was worried labour was slowing), especially towards the end. It's the length of contractions which are most important - 60-90 secs are good contractions (although you might not think so at the time!!). Those long ones are the contractions working hardest to get that cervix open as much as they can, especially towards transition (7-10cms) when the stretch is really on to get to 10cms.