It was measured in drops, a standard unit of measure. When I tried to add white, Scalecoat'sA good place to start with older grey-greens is to assume that it is likley to comprise a basic mix of black, yellow and white which may be adjusted with a few more expensive pigments. The original paint was probably lead based, and lead white would have been a key ingredient.
If the Scalecoat white has a slight blue cast - not unusual as this is a common trick to counter yellowing - increasing the amount of yellow in your mix may help compensate for this. If am am trying to removea blue cast I usually try to work with yellow ochre shades as the pure saturated yellows can shift the resulting colour too far towards green.
I am not sure I can offer much more for Scalecoat, but I do keep a few bottles of Ceramcoat and Liquitex acrylics for adjusting acrylic model paints.