What Stephen said.
One other way to attack it is to never mind the projection; correct the exposure for the room, then replace the projection screen images from a copy of the original Powerpoint and corner-pin the replacement slides into the same dimensions as the screen, which has the advantage of greatly improving the readability of the images over what was originally projected and shot, plus, you have individual control of the image brightness there.
The assumptions there are that your un-obstructed shots with the projection are locked-off and always the same size and proportion, otherwise it may become too much work to keep adjusting the image proportions.
And of course it assumes you’ve GOT the powerpoint originals on file.