From what I've read they need 30% additional fuel at launch (as in a 30% bigger first stage) - that's different than needing 30% left in the rocket after separation. The reason being that to have X [kg] of fuel at Y [m] altitude you'll need (1 + Z(Y)) * X [kg] of fuel at launch, with Z being some function of Y. My gut tells me they'll need 15-20% max for retrieval, the 30% + is just to get it up there. Source: Play KSP.