Let's say the RGB value is xr, xg, xb which range from 0 to 255.

The solution:

1) Render the scene in any way you want with any settings you want, but make sure you use Linear color mapping with 1.0 for both Bright and Dark multipliers and the "Clamp output" option is off, and you are using the Vray camera "Exposure" setting. It would also be handy if you render to the Vray VFB.

2) Measure the RGB value at the pixel you need to match. If the values are, let's say (pr, pg, pb) again from 0 to 255, adjust the Vray camera "White balance" to be (pr*255/xr, pg*255/xg, pb*255/xb) and multiply the camera ISO setting by (xr+xg+xb)/(pr+pg+pb).

Note that this works only for Linear color mapping. In principle it can be done for any color mapping, but the numbers are hard to find by hand.

Best regards,
Vlado

You can customized Balance of white with "white sheet." To do this, you need to create a small white box in center of camera view. Then render its region, and pick color of it. Then put this color in the white balance physical camera.

For interiors desired value for ISO - 400 (less do not desirable, will be dark).
f-number - it is desirable stand between 11 and 16, and if you have pictures of a single object, making it the focus (vase in the interior, the phone on the table, etc.) then the value should be small aperture from 2 to 8 .
Shutter speed - it is the value that can be changed to select normal lighting visualization. (! but only when set the value of Film speed (ISO) and the f-number) its limits from 30 to 500, but it is possible and above or below.

Any pair of settings makes equal brightness. What use does not matter if you do not use the DOF. Adjust brightness using ISO settings.