If you're looking for a meeting place or a good stop around ½ or halfway between Calais and Amsterdam, you can try Antwerp, Belgium, which is about 2 hours east of Calais, or almost 2 hours south of Amsterdam.
About ¼ or a fourth of the way, you can stop at Bruges, Belgium. This is almost an hour east of Calais.
About ⅓ or a third of the way, you can stop at Deinze, Belgium. This is a little over 1 hour east of Calais.
About ⅔ or two thirds of the way, you can stop at Breda, Netherlands. This is about 2½ hours east of Calais, or a little over 1 hour south of Amsterdam.
About ¾ or three fourths of the way, you can stop at Oosterhout, Netherlands. This is almost 3 hours east of Calais, or almost an hour south of Amsterdam.
If you drive straight through without stopping at all, the total distance is 365 km. The distances shown above don't include local roads to get into each town, so if you're planning a long road trip, make sure you factor in traffic and road conditions, rest stop breaks, and interesting detours along the way. If you're in a rush, you can do the trip with zero stops, and your total travel time would be around 3 hours, 36 minutes.