Looking at the Shimano lit (
https://www.manualslib.com/manual/12...o-St-6510.html )
the 6503 # I was recalling was referring to the FD not the brifter which apparently was one of those
usable for double or triple CW setups. So maybe the problem really is the throw of the FD. Brifter is
styled 6510. The adjustment screws on the FD bear against parts of the FD body casting. I was suggesting
a bit of metal removal from the FD body to increase the range of motion. You could back out the screws
completely, disconnect the cable and manually push the FD from one extreme to the other with respect to the CW and see what
coverage range is possible. If it won't go far enough you will need to modify the FD or get a 6503 variant.