I have been down to 220 meters in a type VIIB. You MIGHT start taking damage at 210 but usually you don't get any until 220+. You should not be afraid to click up to 210. Most of the time up to 220 is safe for damage free decent as well.
I have done the same in type IX B/C/41's as well. I once put a B model down to 219 and then went and did grocery shopping. When I returned there was no problems. The game stats say the IX is good for 15 more meters then the VII, but I fail to notice any difference.
I don't like damage on my sub so I havent gotten deeper. It is my understanding though that you can in fact go deeper without mods. Usually at 220-224 you get 1 percent of damage and no flooding.
I would recomend you save the game and dive at flank speed to 210 just to prove it to yourself. Familiarity makes it much easier.