The TTC did a lousy job with these projections, they never really revealed where the numbers came from. I always suspect that when the politicians want LRT (e.g. Eglinton) they underestimate future development and when the politicians want subway (Vaughan subway) they rezone an undesirable industrial area at Jane/7 and claim that many tall buildings will be built, but the area is too undesirable to really be very successful. The other possible explanation is that in the case of Finch/Sheppard, the actual ridership really will be low because a slow LRT line with a transfer in the case of Sheppard is unattractive and most people will drive. This doesn't make sense for Eglinton which is as fast as a subway in the underground part.
To guesstimate projections ourselves it probably makes sense to look at Statistics Canada census tract data which shows where existing population lives in Toronto. You can also get statistics about how many jobs and square feet of office space exists in various parts of the GTA. We know that some fraction of the residents will take the LRT in the busiest hour of morning rush hour, which is what determines how overcrowded a line is. If 10000 people move to Don Mills/Eglinton in addition to about 20,000 existing residents and there is no DRL that probably translates into a few thousand more people using the LRT westbound during the peak hour. Others will use the LRT during other times of day, use the LRT eastbound in the morning towards Kennedy (probably not very many), take the bus, walk to one of the nearby office buildings or drive. Given how few parking spaces are planned in this development not many people will drive to work.