MCP: L/D Ratio and Applied Voltage
As a rule of thumb, apply voltage should be around 20 times of the L/D ratio.
For example if you have a MCP with L/D ratio of 40, the applied voltage should be around 800V (40x20)
For MCP stacking up, you will need more voltage than this. For example stacking 2 MCP (L/D=40), you will need more than 40x2x20=1600V. This could be end up around 1800V to 2100V.
However, in most of the real life case, you will need to tune the power supply and figure out what is the optimal voltage for your setup.