I think applying RLE to the file might give a better estimate, because files have blank space throughout, not just at the end. Just tried it on Super Mario World and the estimated size was 479,154 (468K) bytes.
If you don't have an RLE tool handy, you can force Pucrunch to act as an RLE-only compressor by using the -r 0 switch which disables the LZ compression feature.
If you don't have an RLE tool handy, you can force Pucrunch to act as an RLE-only compressor by using the -r 0 switch which disables the LZ compression feature.