Erm, I don't have vast practical welding experience, but I'd say two decent 30mm long 3 or 4 mm leg fillet, i.e. one in the middle of each of the 2 flush butting sides of that joint should hold it good, for strength - just weld through (over the top of) any tack you've put down. I don't see any point welding round the corners or trying to fill the whole void in where the square cut end doesn't meet the radius edges of the mating member - doing this is just adding loads of extra heat and metal that isn't doing anything. The important point about tacking together is to set and maintain the WHOLE assembly geometry before laying heavier deposits down, so if you have any gussets orbraces supporting the ends of your T assembly, IMO they should be tacked in after the T is tacked together but before putting the structural welds down.