How to judge a Rahu/Ketu Dasa in Vargas?

Dear Ernst,

if I want to judge a Rahu or Ketu Dasa in the Vargas. Lets say Rahu is in a sign of Venus then I judge Venus in the Rasi to see what result this Dasa gives (leaving out other influences to make it easier). Do I judge then the Rasi Lord of Rahu in the Rasi, Venus in the Varga also or do I take the Lord form the Varga and judge that? Lets say Rahu is placed in Saturns sign in Navamsa. Do I then judge Saturn in Navamsa to see what will happen for that persons marriage? Or does one do both, judge the Rasi Lord and the Navamsa Lord?

Kind regards

Narottama