Just to be sure that you don't make the piece to short cut it 2-5 cm longer than needed. The last briefcase I made could have used a few extra cm. so I did not know exactly how long the finale piece should be. Therefore I added 5 cm to work with.
The way you chose to round the corners on the flap have a huge impact on the expression of the final bag. I chose a simple "quarter circle". One thing to keep in mind is to make sure that the transition from the rounding to the straight edge is very smooth. If not it will make the bag look "handmade" in the bad meaning of the word and we do not want that. Also you should mark the center of the front to help position the lock... I forgot to and it made it a bit more difficult.
With the corners rounded the edge can be burnished but only on the actual "flap part" as the sides will be burnished after the gusset have been sewn on. Also the bottom edges needs to be rounded and the strap holders needs to be sewn on just like we did on the front piece.
To determine the length of the back piece close the lock and allow enough room in the top so that the briefcase can expand when full. Then cut the end off. To find the actual "top" of the briefcase you can either estimate it or you can calculate it. To calculate first measure the distance from the "hole" on the lock to the bottom edge of the front piece(17 cm). Then measure the distance from the hook of the catch to the bottom edge of the back piece (54 cm).
What we want is the distance from B to D and divide by 2 to get the position of C.
Length from B to D = 54-17 = 37 cm
Length from B to C = 37/2 = 18,5 cm
Length from C to E = 17+18,5 = 35,5 cm
This means that the top of the bag will be 35,5 cm from the bottom edge of the back piece.
Now we are able to position the top strap "assembly" and make the holes for the screws before sewing it on. I made a major mistake when making these marks. I thought that I would just scribe a line across the entire width of the bag because it would be covered by the top assembly. This if of course not the case. Sure some of the line will be covered by the top straps but underneath the handle you will be able to see it. Damn!
At the same time scribe two parallel lines 2 cm from the center line and connect the ends with a wing divider. Then mark holes with the pricking iron.
When attaching the top assembly do it without the metal bar at first. Otherwise you will not be able to pierce the leather with the awl. Sew it in place and attach the metal bar. Remember to put Loctite threadlocker in the screw to ensure that they don't come loose.
Hold the cutting guide firmly in place. Pretend that this picture was taken before I cut the curve
Side view of the bag.
Now make the same mark on the front.