there were some rather amazing figures in those old DCA charts
Hear what you say.
However, over the years I had a fair bit to do with the old P-charts. While the sums were pretty simple (until JCF's version to cover turboprops), the techniques were applied reasonably conservatively with an eye to not permitting sufficient conservatism to encourage disregard by the end user.
Comparison between specific P-charts and OEM data would require just that so I can't speak to specifics at the moment. However, be aware that some of the early OEM data was not subject to much in the way of independent scrutiny. The P-charts either were based on some simple quantitative flight tests or, for the later GAMA data, some spot points in the OEM schedule.
Unless I had a quantitative reason not to do so, I would happily use the old P-chart as being a reasonably indicative starting point for assessing the suitability of a runway length.