Assuming an internal cable setup

If using an under stem Junction A, then 350mm from each shifter will work on a 44cm wide h-bar (this includes the 180 deg. loop of the cable at the shifter to allow some slack if the shifter gets knocked). I used 400’s as I sometimes use a 46 bar, so I had spare on the current 44. If using the bar end A Junction with the Y split, not sure.

It helps to yank off the crank and look at how your b-bracket shell is setup. Some have a large rectangular slot at the downtube, others might be on the seat tube side of the shell. It helps to confirm as that’ll dictate where you shove the B Junction, which in turns drives downtube cable, seat tube cable, plus F & R derailer cable lengths.

As BTW, I used a 300 cable off the internal battery, then connected to the EW-WU111 WiFi unit sitting in the seat tube, then 350 off that to the B Junction which sits in the downtube above the b-bracket shell. I saw this in a Di2 install YouTube and it keeps the WiFi unit out of harms way. Works on a carbon frame, maybe not aluminum.

It’s no issue going longer as all spare cabling just sits inside the frame out of harms way, where as you are kind of dead in the water if something is too short.