A quick focusing questions - Help!

[bbbdco]

I'm new to the forum. I have a Canon xa h1 camcorder, and will be videoing a stage performance. I know the correct way to focus is to zoom in as far as I can, focus the camera, then switch to manual. That point will be in focus regardless of how I zoom. But what is the depth of field that will be in focus if the person on the stage moves forward or backward on the stage. Any info you can help me with? Thanks.

Dan M.