RatherBeFlying: In an aircraft with an approach speed of 50 kt ( 1.3 x 38 Vso) at full gross
Remember 1.3 Vs is the 50' or threshold speed, not the approach speed. The approach speed should be reduced to this figure by around 50' AGL.
You make a good point about reducing threshold speed relative to weight.
I haven't done the figures for a while, however from memory for the weight changes involved in most light aircraft the change in Vs due to weight isn't significant enough to have to bother with calculating a new 50' speed.
In the case of a DA20 I think 50 KIAS will work all the time, adjusted for wind gust etc if required.