John may disagree with this, but there are those who will say that you can enter the regular charts in the Flight Planning and Performance manual, not with ambient conditions to find RTOW, but with ATOW to find equivalent limiting ambient conditions, and thus an assumed temperature.. .. .Of course, the Flight PLanning and Performance Manual is only of academic interest as its data is so limited. With an enormous amount of time available, you might be able to have a stab at calculating the necessary figures for a take-off, but I wouldn't like to do it often! You'd need lots of data about obstacles too.. .. .Practically, individual charts for each runway (and often, various configurations of bleed, anti-ice, and runway conditions, braking actions etc) will be produced for the operator, by companies such as Flygprestanda, Jeppesen, et al. These charts incorporate a means of finding assumed temperatures for thrust reduction. So, there will not be a '737-300 Assumed temperature chart for reduced thrust takeoffs' per se as requested above.. .. .Hope this is helpful.