I can make the Spectrum/8 Series II work on an old 2007 Dell monitor at 800x600 with some kind of variant SVGA timing with a 40Mhz oscillator in the extra socket. But, I will have to put it on the scope to try to get the correct timing. It shows the classic problem of a green-polluted black level and it won't sync to the Apple monitor. So, the timing is slightly off. The problem with green is that the signals are typically sync-on-green, so if the sync (green) is in the wrong place, it ends up screwing up the black level measurement. With a scope, I should be able to see exactly what signal the card is generating. I am sure my calculations are correct, so something else is happening at the board level (which is not too surprising).
Also, in the process of trying to make the board work, I remembered that custom configs load at secondary init (desktop loading). And, on the Spectrum/8 Series III, the board expects certain oscillators to be in certain spots. So, if you put a 40Mhz oscillator in the extra socket, it will incorrectly detect it as a 57.28Mhz oscillator (Apple 16" 832x624).
It is also interesting to see the horizontal divider at work when tweaking values. For example, when the divider is 8, any change to a blanking value alters the visible screen by 8 pixels (expected outcome).