In normal rides the Drafting Boost increases the draft effect by 50% for 30 seconds (according to various guides found around the web). So in a normal event the Drafting Boost increases the draft to 75% of Double Draft (or Full Draft) levels.
What happens in a Double Draft event? Does the Drafting Boost increase the draft effect by another 50% of the Double Draft?
Normal event, Draft = 1
Normal event, Draft + Drafting Boost = 1 + (1x0.5) = 1.5
Double Draft event, Draft = 2
Double Draft event, Draft + Drafting Boost = 2 + (2x0.5) = 3