[Razor12911]

felice, precomp cannot precompress some files because of the nature/stream of the file, some files has damaged or unreadable streams therefore it is either precomp freezes or it stops working then that error will show up. this errors can be fixed by the improvements of ptecomp versions like Dante said, using precomp 043 but a common trait is the fact that 042 freezes a lot therefore you can earlier versions if 043 fails as well. like 038 and 040, either way, yiu can still force precomp to work if you grab folders or files at a time till you figure out which one causes the error then apply precomp on the other set of folders and files that don't cause the error and apply compression without precomp on them.
compression ratio and best compression doesn't exist because it depends on what a compressor compresses best on a set of files.
a perfect combination of compressors such as this method precomp srep lzma. for example, there can be a best mp3 compressor but if you use it on non-mp3 files it won't work. in other words you cannot expect results everytime you use precomp or any other compressor where they will never work.