This needs 4 taps total: first the 2 ends of the reference card's long edge (the 8.9 × 5.1 cm FishBeacon card, laid flat on the fish, same distance from the camera), then the fish's nose and tail tip. We'll try to auto-detect the card so you may only need the last 2 taps. Print a FishBeacon measuring card. Nothing shows until all 4 points are placed.