Yeah I think a simple divide-by-2 will work well for 832x624 (49.7kHz horizontal).
For Mac 640x480 (35.0kHz) or VGA (31.5kHz) it should probably be 3:2. Or at the least, there will need to be other changes to the flyback circuit to make it happy at 16-17kHz. Longer scan lines means more current and higher peak voltage during flyback, which could damage something. Have you done any experiments running this circuit with NTSC inputs? From previous discussion I take it the transformer itself is rated for it, but there would need to be other changes.
Before any of this, what I may do is run the flyback circuit off a signal generator and see how the CRT voltages change as the frequency goes up or down. I don't have the tools measure the anode voltage but the rest I could check, assuming they will all remain roughly proportional.
For Mac 640x480 (35.0kHz) or VGA (31.5kHz) it should probably be 3:2. Or at the least, there will need to be other changes to the flyback circuit to make it happy at 16-17kHz. Longer scan lines means more current and higher peak voltage during flyback, which could damage something. Have you done any experiments running this circuit with NTSC inputs? From previous discussion I take it the transformer itself is rated for it, but there would need to be other changes.
Before any of this, what I may do is run the flyback circuit off a signal generator and see how the CRT voltages change as the frequency goes up or down. I don't have the tools measure the anode voltage but the rest I could check, assuming they will all remain roughly proportional.