I think the above method by catchlights is just an estimation. The actual reflectivity is also highly dependent on the matrial's surface. There's a reason why greycards cost like $18 and not $0.30. But for normal use, I guess that's the best we can do
You can tilt the homemade graycard a little to avoid reflection, but this homemade graycard may have slight color shift, even a Kodak graycard also need to replace after a being using for a while.
graycard or expodisc is to help you get a neutral white balance, but this may not the white balance that the photographers are looking for, some prefer cooler tone or warmer tone for some scene or another.