^This. 100 percent
While the most common leaded solder alloy has a melting point just under 400 degrees, it's been my experience that your target temp should be quite a bit higher than that, especially if you're working on point to point electronics like vintage tube stuff. When working on some of the heavier wire and component leads I will crank my temps up to 600+ in order to get good flow. Sitting for 30 seconds on a solder joint isn't good for components or old wire insulation, hence the higher temp.
-Steve