I have a grey cardm and imagine it should be able to measure it's RGB values at a certain WB value. Just not sure how I'd then match this to, say a certain Lee Filter to correct it.
With many post processing apps, such as Lightroom, it will show the Kelvin temperature. With Lightroom you could balance both and simply find the difference.
To quote from Rosco’s catalog:
CTO = Mired Shift +167 Converts 5500K daylight to 2900K.
3/4 CTO = Mired Shift +131 Converts 5500K daylight to 3200K.
1/2 CTO Mired Shift +81 Converts 5500K daylight to 3800K.
1/4 CTO Mired Shift + 42 Converts 5500K daylight to 4500K.
1/8 CTO Mired Shift +20 Converts 5500K daylight to 4900K.
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