Spot on Gramps, I've heard it makes plants pretty thin and leggy in veg so if you want short bushes I'd avoid CO2 there, and most folks say to stop co2 enrichment 2 weeks before chop too.
You can buy doors from home improvement stores like home depot and lowes that have some type of a weather strip on all 4 sides of the door. When the door is closed it really is a tight seal. If you have a regular door already you can put thin pieces of weather stripping on the top and sides and by one of those door strips for the bottom.
The last couple weeks of flower the plants arent using the excess co2 anyway,its wasteful unless you have other plants less developed.I set my controller for 1500PPM which seems to have worked best for me and is in line with what I've read.Someone mentioned using 700 PPM thats really not going to accomplish much IMO.The co2 levels indoors are nautrally higher I run over 600PPM in my house.I would go with 1000PPM at the very least but 1500 seems to be the magic number.I have a door flap on the bottom of my door but to be honest I dont think it was really necessary.I wasnt losing much co2 at all through the cracks or the bottom of the door.I never considered a "sealed room" to be hermetically sealed if you know what I mean.Your controller will let you know what is up in your particular situation.As for co2 in veg if it were one or the other,I'd definately go with co2 during flower.I've never used it in veg but have never really seen a need to.Usually I am trying to slow my vegging plants towards the end of veg waiting for my 60 day flowering plants to finish.60 days can veg a cut into a monster without co2 enrichment
Spastic I keep my flower room at 1500PPM until the bitter end for my plants because I have other plants in there that are only 30 days in.If you ask me it total bunk saying the excess co2 somehow will be detrimental to your finishing plants.Like I said they arent really utilizing the extra gas but its certainly not harming them.I guess thats one of those internet things that if you say it enough,its true lol.