In new to this, but from what I’ve read on this forum, downstreaming is almost never used for roof cleaning, because roofs require a much hotter mix than siding. It seems like people are using about 5% SH on roofs (someone please correct me if that is incorrect). Downstream injectors pull a ratio of somewhere in the neighborhood of 10:1, the hottest mix being around 8:1. With 12.4% SH and a 10:1 dilution you’re only getting about 1.2% SH on the roof. Too low for a roof, but fairly hot for siding (assuming people put roughly 0.8% SH on siding, again please correct me if I’m off on that).
If you have an XJet siphoning 12.5%, and your pressure washer is no more than 4 GPM, you might be able to get the mix hot enough for a roof. See this video about that and compare with what others say: https://youtu.be/G-UGMCdHZa8