setting up in a room that I've always had problems in. I've read a theory, that to find the ideal location for subs, is to place a sub where you want the bass to be (in the audience). Then play a tone and walk the room. Where you find the bass to me the best/most intense, put the subs there. I tried it, it worked. Used a few tones to make sure it was good for a the whole range of bass. Funny thing though, the corners were not the ideal location (which was where I usually placed them). Also found that a V-plate was not ideal either, but that lined up facing the wall was best. So try both ways.
In conclusion, if you have an hour to kill/use before the show, try it out. You may find a position that is better that you wouldn't expect. Measurement and math would probably work just as well, but this was fast and fun with friends.