Can this be extrapolated to seamlessly handle saves from 1399-1453? I don't want to limit users to only converting in 1453. If you can write that up for me, I'm try implementing it so we can see how well it works.
I'd argue for leaving those standards fixed and have even western nations need to "westernize" out of the medieval mindset in early conversions if they'd failed to achieve the tech levels historically associated with being in the western tech group.
However, the "historical" CK2 tech levels vary linearly between the start and end techs, so you can interpolate easily for any year to find that year's baseline thresholds for each group.
Catholic 1066 avg - 1.0, 1453 avg - 3.5
Greek 1066 - 6.5/3, 1453 - 3.5
Pagan/Slavic Orthodox 1066 - 0, 1453 - 2.5
Muslim 1066 - 6.2/3, 1453 - 3
So my proposed variable mapping to EU3 groups:
Catholic & dejure HRE westward = western if tech average >= Catholic tech for that year, else eastern
Greek = western if tech average >= Catholic + 1, else eastern if tech >= muslim, else ottoman
Pagan, Slavic Christian, east of HRE = western for tech >=Catholic + 0.5, else eastern for tech >= pagan/orthodox, else ottoman
Muslim = western for tech >= Catholic + 1, else eastern for tech >= catholic + 0.5, else ottoman for tech >= greek, else muslim