I think that's way to high.
The ISO recommendation for softproofing is a contrast ratio of not more than 1:250.
"...... a monitor contrast ratio of approximately 1:250. This is ideal for soft proofing because it
simulates a rich glossy print’s density range of approximately D2.40 (Dmax 2.50 - Dmin 0.10) or
8 EV (2.40 ÷ 0.3 = 8 ) which equals a contrast ratio of 1:256"
And a 2.40 dmax translates to an L* value =3.6, which is what you'd want your monitor to display with softproof viewing turned "on". It's not the native contrast ratio that the monitor needs to be able to reach for softproofing to work well. A subtle fact about most monitor profiles is that they remap the measured blackpoint of the monitor (sort of like relcol with BPC), no matter what the black level is in reality,so that L*=0.0 always gets assigned to RGB 0,0,0 in the display profile. (Note: ColorEyes display Pro does have an optional "absolute" rendering mode for monitor profiles which will map RGB 0,0,0 to the actual measured monitor blackpoint L* value , but that's an exception rather than the rule).
Thus, the best monitors for softproofing will require a black level which approaches L*=0, not 3.6, and then the monitor black will be increased ever so slightly above that black level to the 3.6 L* value when the softproof mode (in PS, for example) is turned on. In practice, L* = 0 isn't attainable, but to get to an L*=1.0 level on your monitor, for example, you need a contrast ratio nearly 1000:1, and better monitors can achieve this level if you also take the precaution to shield stray light from hitting the screen. Once you achieve it, and you then open a digital file with both RGB 255,255,255 whitepoint and RGB 0,0,0 blackpoint, the PS info tool will dutifully tell you that you are seeing L*100 for maximum image white areas and L*=0 for maximum image black areas, but your monitor is only getting down to approximately L*=1.0 relative to the monitor white in reality. Not bad at all but not perfect.
For poor monitors being used in lighting conditions that compound the already poor screen contrast problems, it's not unusual for them to reach only L*= 4 or 5 at native black level relative to monitor white. Then when you turn on softproofing, the predicted shadow values get lifted on a relative basis above this L* error level, so the softproofed image ends up looking flatter in contrast than the actual print. This situation accounts in part for the "makes my image look ugly" complaints many people express about soft proofing. Bottom line: you need a good monitor with high native contrast ratio to get the best softproof. It will have to have a native contrast ratio of 750:1 or better still.