OK, for a 25 lb projector, this isn't needed. If you didn't have sheetrock, then sure why not "do it right." But with sheetrock there's no way I'd sweat it for 25 lb. Just use a small mounting plate screwed across two joists and a few screws into each. Not only would that be fine, but the lower flanges are resting on sheetrock screwed to an even larger area further distributing the load.
That being said, for reference, you'd want at least the solution seen in the pipe support image on this page.
Better yet, span from bottom to top flange like the backer block in this pic
The idea is to support the weight by the web at worst if it can't be supported from top flange.