When figuring out the temperature and timing, I find it helps to hold the iron in one place and count out five seconds, then remove the iron and roll (or press) the spot as it cools. Then check underneath for melt-through before ripping the weld apart to see if it’s bonded fully. If five seconds isn’t right, try six (or four, etc.). If no amount of time consistently lands you in the sweet spot, you probably need to adjust the temperature.