How can I put the understandability of classical Chinese and modern Chinese into scale?
in short, it's middle english.

a long explanation, in general, roughly:

i would define classical chinese as "text from bc770 to bc221 (春秋 - 先秦),  afterward, it's literary chinese. there're big differences between this two, mainly, the grammar, style, vocabulary &amp; underlying assumption.

for classical chinese text, thought modern average people know most of the characters and words, they would have great difficulty to understand an article without help. if you ask them to write in the same style. it's no.

it's like asking a land buddy to read the bowditch

for literary chinese text, modern average people would have difficulty to understand the articles, they may guess the meaning of words, clauses. if they've help (dictionaries, mentors), they could grasp the meaning roughly. i guess there're very few people can write literary chinese now.

it's like asking a high school children to read university level textbook.

next, types of articles has huge differences in the comprehension.

poems, correspondences and novels are easier to understand; while religious, medical, astronomical related materials are, very very tough.

have fun :)