🚀 go-pugleaf

Does anybody out there in netland  know the maximum allowed sync.
frequencies in external sync. mode ?  I  have made  a calculation
that goes as follows: 
 
The  ST  does  2x10^6  video  and refresh  accesses  per  second.
The typical refresh cycle for 256K and 1M DRAM's is 4 ms,  or 250
refresh cycles/second.  There are 256 (256K) or 512 (1M) accesses
in a refresh cycle.  Thus,  there are  at most  250x512=1.28x10^5
refresh accesses/second.  That leaves 2x10^6-1.28x10^5=1.872x10^6
accesses/second for video. There are 80 video accesses needed for
1 scanline  (320 pixels, 4 bit/pixel or 640 pixels, 2 bit/pixel).
Thus there can be no more than 1.872x10^6/80=2.34x10^4 scanlines/
second, ie. maximal horizontal sync. frequency is 23.4 kHz. There
are  200  lines/frame,  plus  some  blank  lines  (say 50) in the
vertical retrace period. This would mean a maximal vertical sync.
frequency of 2.34x10^4/250=93.6 Hz. 
 
Does anybody know  if these figures  are correct ?  Are there any
other factors which limit the maximum sync. frequenies ?