I just tried to find a closest distance between sketchpoint and face by using ModelDoc2::ClosestDistance with this:
Object pointA, pointB;
Double distance = (Double) Doc.ClosestDistance(Plane, point2Check, out pointA, out pointB);
return Double * 1000;
Above function just doing great, except some problem for sketchpoint that lies on the same plane (that correspond to the face).
By using the test_assy model attached here, I found that #plane5 still has a distance even though they (sketch point and face) were on the same "virtual" plane. Because, I tried to smart dimension the sketch point and the face (that correpond to #plane5) manually, but nothing came up.