Every set should be taken to near failure. For the cable crossover, alternate stretching and then flexing your pecs between sets. So you’ll complete a set and then stretch for 30 seconds, then do another set and flex for 30 seconds. After the stretch/flex, you can rest up to 45 seconds. For the stretch, rest your forearms against a doorframe, or use the beams of a power rack, and lean forward. To flex, tense your pecs isometrically.