The best way to measure to your current Archies Flip Flops is to do the following:
Use a ruler to measure on the underside / grip side of the flip flop, NOT the top of the flip flop.
Lay the ruler along the length of flip flop using the marking points provided, ensuring that the ruler is firmly pressed against the flip flop and the flip flop is fully stretched out.
Take the measurement from the furthest point of the flip flop that you can see (this point is on the top of the flip flop as the underside / grip side is slightly angled in). Please be as accurate as possible and measure to the nearest mm.