600 Ohms is the nominal *impedance* of a telephone set. The DC resistance is necessarily less than 350 Ohms. Common values are roughly about 200 Ohms.
Loop current is supplied through a resistance though, which is typically 400 Ohms. (For extended length loops the voltage may be higher than 48 VDC and/or the resistance may be less than 400 Ohms.)
And it is also true that the line has to work when the battery voltage is low! So while it is a "48 Volt Battery", the typical voltage is 52 VDC and the range that may be seen is 42 to 56 VDC. So maximum loop length would need to be calculated using 42 VDC, not 48 VDC.
Also the common wire sizes are 26 gauge and (on rare occasions)
24 gauge.While 20 mA is the current at which the line is considered "offhook", the actual steady state loop current must be 23 mA or higher. Typically it is 27 mA or higher (and is current limited).
Hence you have the right idea for determining maximum loop length, it is actually slightly more complex and requires different numbers.