YES…in response to the original question, YES, it is seriously risky to simply assume that you may reduce thrust (by any method) if there are no charts, electronic or otherwise, which would provide the correct numbers. Even full N1 (or EPR) without the same information is risky, despite all your experience. So my answer again…YES…btw, I flew at a private Op (heavy jet) once with a guy who thought the numbers provided from the FMC were perfectly adequate and the reductions in thrust were arbitrarily applied at every airport we flew out of despite the requirement to have proper performance charts. I left…he got fired! End of career…for him!