You unfortunately can't test 100% accurately on the tool since QR can't be your first segment. Just for testing purposes, I might try OSL-MAD with a stopover, then do the rest of your itinerary as you'd do from OSL. LHR isn't a good one for testing because of APD and you might attract some stray BA YQ, and QR doesn't fly their own metal into HEL, so not sure how that affects things since the tool would only show HEL-DOH on an AY code.

I don't think QR has the giant fees that tend to get rumored around here. I built a QR heavy ex-CAI during the sale and wasn't paying a significant amount more than others posting different itineraries. I was also told by QF when I did an amendment that CAI-AMM-DOH-CGK (RJ, QR, QR) had higher YQ and taxes than CAI-DOH-CGK (QR, QR), and that difference nearly offset the entire change fee.

Just my own anecdotal findings, the ones that may increase YQ more than others, BA, QF, CX, and MH. Having said that I've seen some others post itineraries with BA coded TATL or to Asia that didn't get hit with a huge YQ, so YYMV. As far as I understand the ticketing carrier definitely matters, so those may have been QF ticketed and didn't carry the same YQ implication for BA codes that an AA or BA ticketed itinerary would have. I've also come to an understanding that it's bad to have QF codes unless it's an ex-Japan itinerary (where YQ doesn't matter).