TSFC also increases with lower throttle settings.
TSFC is actually quite complex. And we have to keep in mind: TSFC is no physical value, it is simply fuel flow divided by thrust. Sometimes it can be misleading. For example: TurboProps have low TSFC, but the lower cruise speed might result over higher fuel burn for a given distance (depends heavily on some other variables, too).
But Flight Performance is always fun.
But Flight Performance requires good data, for aircraft drag and engine performance. Both are hard to obtain (but are available to some extent). With an Excel-Spreadsheet however you will probably run into problems.