The mechanics of the FP is actually quite simple (as opposed to the over-complex civ3 FP). When a city calculates distance maintenance it just uses whatever palace city is closest. Number of cities maintenance is unaffected. So we just have to place the FP so the total amount of palace distance (sum of all cities' distances) is at a minimum (this holds true as long as we don't take populations into account as that affects distance maintenance but since we have a fairly normal distribution it shouldn't matter too much, except that it might be a small extra argument in favour of placing the FP among the medium/big cities we capture from Loco).
edit: but off course, in the end, nothing beats WB tests, so we might want to do just that before starting contruction of the FP.