Eek, ladder
. The bridge length is indeed what you are comfortable with.
How I measure (and was taught) the length of rope I need for my prussic(friction hitch) is; you grab the end of the rope and extend your arms as far as you can comfortably and cut it off to the length of what your arms can go. I guess for an open end hitch(blakes) you would extend them out as far as possible then take half of that(I used to guesstimate it when I used the blakes)?