The value in register pair 6802h/6803h is the first displayed line and not the start address of the 16k block.
The internal comparison used :
SPLT7 SPLT6 SPLT5 SPLT4 SPLT3 SPLT2 SPLT1 SPLT0
VC4 VC3 VC2 VC1 VC0 RC2 RC1 RC0
SPLT is the programmed value. The index denotes the bit number.
VC is the internal CRTC vertical line counter. RC is the internal CRTC raster counter.
The value therefore will wrap at 256.
Also, during vertical retrace, the value in register 6801h should not be set to 257 less the total number of scan lines on the screen.