Karin:
To my gesso, I use a golden mean ratio - 1 part marble dust to 1.6 parts gesso.
If you were to use it in actual paints, then I think that following the generally accepted 20/80 ratio that is used for mediums - 20% marble dust to 80% paint might work out ok. I know you already know to try it on a test panel first.
Many of the thick impasto effects we see in old master works, I believe, were likely built up over repeated applications during a number of months. If you just apply an impasto layer directly and it is thick, then you risk cracking as the top layer will dry while the underside will take several months if not a year or more to dry. When it does, and over time, it will contract - at a different rate than the top layer - and it is almost inevitable that cracks will result.
Now, the addition of a dryer such as liquin to this mix might help mitigate the risk of cracks as the layers will dry faster, but I still would be tempted to apply several times to ensure that things below were dry - my opinion.
Hope that helps.
