I use the same technique. Works great. My to cabs exhaust into attic and pull about 350 out of room (10x10). You can easily fell the draft coming all around the room door, and if you crack the door open ,it will blow open about 6in. Major neg pressure.
I am now experimenting with a couple of ways to release some of this pressure so it doesn't effect the heating and cooling of the house
Since I have been doing this, cpl of places in the house stay cold and breezy.
I want to be able to leave the door shut and know that the room is getting plenty of air. Running a duct from other end of house to the room and leave it passive. Even things out a little. While doing this I will still need to maintain some neg pressure in the room for the door. So I'm trying a very small duct.
Always something to do or try.