a drawing, photo or more info would be helpfull...
if the two pieces are laid along side each other and the box has slightly rounded corners then it should be OK. You just need to make sure you push the weld right into the root.
If one piece is going up at 90 degrees from the middle of the other then I'd just tack it on with one spot, set the angle at 90 degrees, check that it's in line on the 'other' edges then weld around it starting on the side opposite the tack.
Corner to corner I'd look at 45 degree angles on each piece.
Its 25mm box, and the vertical piece is coming up from the middle of the horizontal piece.
Its more the technique for welding into a corner I'm looking for - what sort of torch movement should I use rather than getting a nice flat weld bead at 45 degrees to the 2 pieces I'm getting a blobby weld, I'm obviously doing something wrong...
As I wrote previously, one spot weld in the middle of one of the 'faces', true it up with straight edge and set to 90 degrees...
then weld the full face opposite the spot weld and continue around the corner to do the full second face.
Flip it around and do the other two faces going right over the tack. If you don't want the slight bulge at the tack you could stop as you get to it, and re-start on the other side of it. With practice you could get the weld pool to melt into the tack.
I used to leave a small gap around 1 - 2 mm between the two pieces and it would be done 'by hand', ie, I'd hold the vertical piece in my left hand as I tacked it to the horizontal piece. This gives you the penetration and also allows side to side and back and forth movement to set the angle, etc.