You’re close, after a certain unitID only class growths will be used for autolevel. The FE8 decomp clarifies that unless the unit has the Boss flag set in their character data, units over id 0x40 will use class growths and units below that id will use personal growths.
If you want to change the “units at this ID or below use personal growths unless boss” ID, write the byte you want at 17C4C over where the 0x3F is.