How far away is that Pied-billed Grebe?

We went to the remote Loch Feorlin in Argyll to find this Pied-billed Grebe from America. Last month I had worked out that the field of view in my telescope is worth 1 degree or 0.0175 rad. Last time I didn't photograph the bird. The water was calmer this time. Measuring the screen, the grebe therefore subtends 2/80 x 0.0175 = 4.36 x 10^4 rad. The grebe is about 30cm long. Angle subtended in radians = arc length/radial distance. So distance to the grebe = 0.3/4.36 x 10^4 = 690 metres. Map measurements suggest 400 metres. So at least 50% uncertainty in this method.