Here's two opinions:

>BBDSOFT app note

As you can see they just kept dividing length by two as rates increased by 2.

Then their's MAXIM/DALLAS:

"A final limitation to discuss concerning RS–232 communication is cable length. As we have already seen, the cable length specification that was once included in the RS–232 standard has been replaced by a maximum load capacitance specification of 2500 pF. To determine the total length of cable allowed, one must determine the total line capacitance."


Maxim/Dallas App Note


You might consider looking or making line converters or boosters although most boosters don't run at your intended rate.

There is quite a bit of info on the WEB search GOOGLE with: RS232 Maximum Cable length 115200

regards,
p