How do i find the center of a speaker front plate ?

Frightening. Don't they teach math in school these days? Measure from one side to the other then divide by two. I use a combination square and a pencil to mark the lines. I also never mount drivers on center. I make them offset and "bookmatch" the pair of speakers.

Use a measuring tape. Mark the half on height and you've got the center.

Raw drivers come with complete spec sheets, and often, hole templates.