I document all of the various equations in this book (https://www.amazon.com/dp/B088K38Q9D). All of the software is free online here (http://dudleybenton.altervista.org/software/index.html) There is a free Excel AddIn for steam properties (http://dudleybenton.altervista.org/software/AllSteam104.zip) The AddIn includes the 1967, 1969, 1984, 1995, 1997, and 2020 formulations and extends up to 6000K and 150 GPa. If you want the properties outside of Excel, you can call the library from C. There's an example on how to do that in the archive. If you just want a curve fit, you can calculate the range you need in an Excel spreadsheet and then curve fit it. There's a free curve fitting tool at the second link above. It will create a function (VBA, FORTRAN, or C format) that you can paste into whatever code you're using. If you need any help just send me an Email.