They work very well, perfectly in fact.
I have two Wallace Expo Discs, which were purchased last century. My initial use was for colour accuracy for colour printing of my C41 colour negatives. When using them over the end of the lens and pointed directly to the light source, they agreed with the readings from my Gosson Profisix light meter
The wallace expoa Discs I have, are a push fit over the lens, so straight on, meter, or take an exposure, the pull off and use the settings indicated. Fast and accurate.
Wallace has since died, but his daughter realised that what worked colour wise for colour negatives, worked perfectly for electronic cameras. If you search on the web I'm sure you should find them. the wallace discs were colour accurate and calibrated with a certificate stating how close they were and what if any deviation there was/is.
I believe there are imitators on the market, I have no idea how good or bad they are.
Mick.