My bet is that MePlat has already made a computation and has what he thinks is the answer - he's just trying to reel you boys in. Of course, I COULD be wrong....
This was my thoughts also but I did like the question. On the surface, Graybeard's reply#3 would certainly seem to be in the ballpark. (1" @ 10yds
should=3" @ 30yds) but to me, a 3" seems to be larger than a quarter even @ 50yds.
If I really wanted to find the answer I would arm myself with some construction paper & sissors and cut a strip to match the edges of that 10yd quarter with my arms being extended and a rest avalible for my wrist's.
The strip would then be used with the 3", backing that disk off until the edges matched.
This answer came to mind to me yesterday when I had my arm straight out with the thumb sticking up and using the top of the nail to gauge the distance of a doe (based on chest width) that had a good 800yds of open ground behind her. This is known as the poor mans range finder but it is consistent to a particular object at a yardage that one knew to begin with.