eCalc C rating is as I understand it - the actual USED C rating ... not the battery label ! If eCalc gives 15C as minimum used ... then you need to look at having a safety margin on top ... I would be looking for at least 25C to give required 15C.
I used to go for minimum C rate but ended up replacing LiPo's too quickly as they degraded with use and their real C rate dropped off ...