Length extension attack SHA-1 message division with padding problem?
OK, lets go through this step by step:


the message size of http://example.com/downloadfile=report.pdf is 10 bytes, as only the filename (report.pdf) is hashed according to the article;
the key size is usually known and was explicitly set in the article to 11 bytes, not 15 (11 bytes is a bit short for a key size, by the way);
so the length of the message is 10 + 11 = 21 bytes, or 168 bits and A8 is indeed 168;
to the padding + length encoding needs to be 64 - 21 = 43 bytes, lets check that:
%80%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%00%A8
yep, that seems to be correct.


Note that 64 bytes is 512 bits for the block size and that e.g. %80 is the encoding of a single byte with value 80h, so you need to count the characters and then divide by 3.

So I'm not sure how you got your input values for the message size and key size, but they are way off. So you would not end up with the right values, regardless of the correctness of your formulas.