when you have BF and UBF data---but no optimum data?
OK .. optimum is what the point of the calculation is all about. When you start with a particular runway, you don't know what the optimum is going to be .. although, for a Type with which one is familiar, an educated guess is not out of the question ..
(a) first determine the goal. Is it maximum weight ?, a certain pad of (ASDA-ASDR) ? ... whatever
(b) second use the AFM charts or software, as appropriate, to iterate TOW for the conditions until the goal is achieved
Repeat this for the range of conditions (usually OAT and W/V) desired
The specifics of how one goes about the actual calculations will be determined by the presentation in the AFM or the degree of flexibility associated with the software input.
Am I getting closer ?