Question about engine bracket installation

[Privateer24]

Hi BD members, I have a 24' Privateer powerboat. When I purchased the boat it had a fiberglass engine bracket that extends 30" back from the transom. It had a 25 in shaft motor on it before, but I am installing a 30" shaft motor and wanted to see if someone knew of a formula for measuring the transom for placement of the bracket. I was tempted to just measure 5 inches up from the previous installation but I'm not sure that they did it right so I would rather do this from scratch. Important holes Im drilling through a newly refurbished transome and want to get it right. Any info is much appreciated!
Martin