I apologise for being negative, but rather don't try to design an LFE yourself. The reason for using an LFE is to get improved flow measurement accuracy over a wider range, but an "own design" unit is unlikely to achieve this.
In theory the pressure drop is easy to calculate using standard laminar flow piping pressure drop procedures. But to know exactly what the inlet and outlet effects are is difficult. Rather buy a purpose made unit from one of the many suppliers who have done the hard work of setting up the calibration charts.
