Taipan 28 - How well offshore?
If the Taipan is a close sister to the Newell Cadet, then it is in fact a fin keel (or close to it) by the classic definition of a fin keel. These days people seem to think that any keel with an attached rudder is a "a modified full keel". In the era that these boats were being designed and built, they were considered fin keels with attached rudders. The definition of fin keel during that era was any keel whose bottom was less than 50% of the length of the boat or the length of the sailplan (which ever was the longer). Considering that your waterline is only 70% of your overall length and your rudder post rakes forward and your stem rakes aft, I am reasonably certain that your keel is a fin keel(or close to it) with an attached rudder.
Again, as I have explained, without doing a curve of the center of buoyancy at various angles of heel, and without finding the center of gravity, I know of no surrogate formula that would even closely approximate the LPS (AVS). The one that you were using is clearly in error big time as boats of the general type and design to your own generally had LPS''s around 120 degrees or less.
By modern standards 4''6" draft on a 28 footer is quite shallow. It is true that older designs often were quite shallow. Both the Triton and H-28 used cast lead ballast which helped a little but compared to more modern designs both are quite tender for thier drag. The Triton, like your boat, has a fin keel with attached rudder and is a miserable boat in a blow. In the case of the H-28 she is a nicely modeled hull which greatly helps reduce drag and a low aspect sail plan which helps reduce heeling, all and all a very wholesome design.