I'm check this calculator and I can said that this simple calculation isn't correct or not agree with EM simulation and measurements.
Anyway, LPDA structure check firstrly in any wire based codes(MOM) and on the end in full 3D based. Impedance is very critical(wire method isn't correct) and because use full 3D simulator.
What I am getting is the longest element is just resonant at the low frequency end and the far end of the array is resonant way above the highest frequency. I have tried radically different frequency ranges and tau values
I'm trying to build in domestic conditions LPDA antenna like on the attached picture (Lectrosinics ALP500), but am having some difficulties with the measurements.
Also, I checked your excel formula, but it's designed for tube LPDA.
Could you, please, help me with correct sizes and proportions for this model?