estimating entropy of password
If you have a rule such as "substitute 5 random letters with a fixed symbol" a direct calculation might be possible if you know average word length.

If you can't enumerate all the symbol substitutions into each word via a rule, it will be difficult to calculate directly. 

A reasonable way estimate entropy when you can't calculate it is to use data compression. Write a script to output 1 million passwords generated using your method to a text file. Then compress with a high compression, slow algorithm such as LZMA on maximum settings. Perhaps use the 7-zip utility and try all of its available algorithms, picking the smallest result. The size of the resulting file in bits divided by the number of passwords in it should be pretty close to the entropy per password in bits.