You gotta leave enough room to react and stick out the side just a bit so you can see in front of it just a bit but also you can stop a lot quicker than them.

I generally don't do it with Semis though, and I probably don't get as close as the people you're thinking of. If I'm on a long trip I'll ushally try to find a fifth wheel camper going roughly the speed I want and I'll set my adaptive cruise to the minimum distance. The campers throw fewer rocks.

Also if you're ever drafting make sure to keep an eye on your temperature. Most of the time it should be fine but a hot day and going uphill it could very easily overheat, especially if you've done some of the other hypermilling mods like partial grille block.