2 into 1 purge tower retraction settings

[phongshader]

I’ve been able to get prints that don’t string with a retraction around 3.5mm. However when I do a multi material print I get stringing between the purge block and the part. To get rid of that stringing I have to up the retraction to 12mm. Is there a way to setup S3D to only retract 12mm between the purge block and the part? There is a tool change retraction setting under Other in Process in S3D, 36mm retraction in my case, but that is for actual filament change. There is no tool /filament change going from the purge block to the part.