When I can, I run a pair of box fans exhausting through the windows in one room, letting the air in through the windows in the other room. This feels roughly like living on a houseboat, and is how I can read 450ppm.
I'm now at 728ppm after cooking lunch, windows closed. There's a good kitchen exhaust to a roof fan, so I can get that number down a fair ways by cracking a window.
Showering for fifteen minutes in my last apartment would push the bathroom CO2 up to 2000ppm, even with the stock weak exhaust fan on, and it would drop slowly for twelve to fifteen hours but never below 1200ppm so long as someone was home. 750 sounds like luxury :)
I'm now at 728ppm after cooking lunch, windows closed. There's a good kitchen exhaust to a roof fan, so I can get that number down a fair ways by cracking a window.