Since you are starting with a design for which you don't have the specs it would be wise to do an autocollimation Foucault test with the front lens to find out what it's color error is. Do this with 3 color LEDs is good enough but more colors will help. Generate the curve over the spectrum and you will have a good idea of what you need to correct for in the second doublet.
I might note that the design in that post to the ATM list wasn't all that specific as to the best design but rather is a common design that is fairly appproximate when the glasses aren't fully known. Raytracing will provide the difference between a design which will provide a decent view like a Clark, etc. telescope and the much better view of what you have with your more modern doublet. There's a lot more improvement as you have seen with Nagler's work and, with the modern raytracing software, it's not all that hard to equal his work if you work carefully.
I might also note that there is a "color corrector" design that has been done by the russian importer and have heard some good reviews (reviewers are not really known to me and could be a shill for the guy) of the corrector.
There's a lot of work that can be still done with refractors as the professional industry has gone exclusively to reflectors for their needs and thus have no need to drive the optics designers to do their work with refractors.