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

Lights spots on the ceiling - the best way to get rid of them, it place the land behind the window. I mean to put outside the plane near the window with a dark (the more darker the less light on ceiling) material. You can give a little color and color of light on celing will be the same.

VrayLight in portal mode MUST have put on the outside of the room, and not internal, as is the normal practice. The noise will be much less. Well, and of course subdivs for portal mode (which means primary light will be either Sky or HDRI) will need two times greater than for normal VrayLight of the same size.

For quick Exterior preview rendering, you need to put Vray Light in a mode Dome Area Light type and put in to Texture Sky (image, hdr or shader). Other settings are not necessarily touch. In this case, no need to include the GI, and you can see quite quickly lighting and materials. This is how it will turn out, and render final. But! Will be noisy and not very fast. And most importantly - too deep shadows. Making a contrasting picture easily. We should only reduce the gamma in color mapping.