measuring distance between objects of known size in photo

Hi. I'm wondering if it's an acceptable method of measuring distance between two objects of known size by averaging their scales. In my case I have a photo of birds on the water. Each bird is known to be about 37cm long. I want to find the distance between two of them on the photo, the nearest one is at a scale of 1cm:37cm, and the second is 0.5 cm:37cm. On the photo, they are 7 cm apart. I've figured the average scale is 1:56 (1:37 plus 1:74 / 2) for the two birds, and have computed the distance between them to be 392cm. Does this sound correct?

In my case the distance doesn't have to be exact, so if there's an adjustment for vanishing point issues, as long as it's small, it doesn't matter, but please do tell me if such an adjustment is required (i.e. the two birds are off to the side of the photo), if you can.

I also tried a "small angles" measure using Kinovea tracking software to determine the angle, but the two methods didn't give me close to the same measure. I suspect I can't use small angles unless the two birds are directly in line with the photographer, which they aren't, but I'm not sure. Are there any other good methods of measuring distance between
objects of known actual size on a photo?

Thanks for anyone's help!

Hugh