It is not a simple integer constant because the Z80 and NICK are clocked by separate crystals. For the NICK chip, the clock frequency is derived from the PAL chroma sub-carrier (17734475 / 4 Hz), such that one line is exactly 284 cycles of that, which is slightly different from the PAL standard. Thus, the horizontal refresh rate is 17734475 / 4 / 284 = 15611.3336 Hz in theory, instead of the standard 15625 Hz. Although the circuit that generates the clock can be broken in some of the machines, resulting in an inaccurate frequency and "waving" picture or other artifacts.
One line is then divided into 57 "slots" or characters, at a frequency of 889846.02 Hz. In each of these, the NICK chip can read 2 bytes from video RAM, while one byte access is allowed for the Z80. In the first 8 slots, the current LPB is read (16 bytes), then 46 slots are available for displaying graphics, and the last 3 slots are reserved for refreshing the VRAM (6 bytes). If a "refresh" slot is not border, then display is generated from the last valid byte read. The NICK input clock frequency is 16 times higher (14237536.27 Hz), this is the dot clock in the highest resolution mode.
So, on a standard 4 MHz machine, the number of Z80 cycles per line is 256.224, not an exact integer, and it may vary randomly by a small amount. I think the 8 MHz crystal for DAVE/Z80 has an accuracy of 0.1% (Zozosoft may correct this information), the NICK frequency derived from the PAL crystal is better, at least if the clock generator circuit works correctly.
Any access from the Z80 to VRAM or NICK I/O ports (80h-8Fh) is subject to clock stretching, the Z80 clock is halted until the phase within a slot reserved for the Z80 is reached, and there is some additional wait as well, so a VRAM access can be delayed (at 4 MHz) by 1 to 5.5 Z80 cycles in half cycle steps. If accurate synchronization with NICK is needed, then this can also be useful, like in the
fbscroll.com demo.