The CMYK values here are built from whatever CMYK color space you set in your Color Settings (from the source RGB). New document might be that or antoher CMYK profile you select.
As Andrew says: the result of the conversion depends on the RGB-profile and the CMYK-profile
Additionally on the Rendering Intent and Black Point Compensation BPC = On or Off.
I think, the question refers to K=90, which means 10% of totally black, therefore RGB should be
something like 25 in each channel. Both assumptions are considerably wrong:
Printing by K is a affected by dot gain: printed dots are larger than defined by percentage.
Coding of RGB is done by gamma encoding. Thus, a mental prediction is not simple.
We can do a test:
Use in Color Settings RGB=sRGB and CMYK=ISOCoated-v2-eci (or any profile for European
ink and coated paper). Renderin Intent=Relative Colorimetric, BPC=0n.
Load page 11 of this doc in Photoshop in mode CMYK:
Set indicated colors in Info: CMYK and Lab.
Verify for any swatch that it is K-only (C=M=Y=0).
Set indicated colors in Info: RGB and Lab
Measure swatch K=90:
R=G=B=59 (values are a little different because of small numerical errors, going through profiles)
L=25, a=0, b=0 (small errors as well)
Measure swatch K=100
L=9, a=0, b=0
Black ink K=100 does not deliver absolute black L=0. Therefore we don't have R=G=B=0.
So we can easily find Lab and RGB values for K-only values. The doc contains as well
information about gamma encoding and dot gain.
Best regards --Gernot Hoffmann