Well on regular keyboard i’ve been using 28 gauge wire of over 10ft without any issue. The problem really seems to be in the power requirement of the HHKB (Topre boards in general I believe) The USB Hub is 3.0
On a side note, it’s important to say that the cable doesnt flat out not work. It works but every now and then (like 15-20min I get a quick disconnect and then it reconnects. The lenght of the cable might still be slightly too long for the gauge of wire used. I’ll try over a powered USB hub since that might help over come the extra wire from the USB hub itself.