I agree with DGXL concerning the detail of this joint. There are more guidelines shown in AWS D1.1 :2004 Section 2, paragraph(s) 2.7.1 & 2.16.1.1 -Structural Welding code to help with this.
Also see AWS D1.1:2004, Figure 2.2
(A) If welded from one side and has two different thicknesses and you transistion the weld surface at 2.5:1
or (B) you can use welding and chamfering at 2.5:1 of the thicker piece
or you can use (C) If you want to chamfer the thicker piece at 2.5:1 as DGXL explained.
John Wright