Scrypt as a KDF with one-time high-entropy input
If you have a high entropy input, then scrypt isn't a good choice. It's purpose is to compensate for the low entropy of a password. Don't ask the user for memory/cpu factors, you don't need them if the input is high entropy. You don't need a salt either.

Simply use an input of at least 16 bytes from a secure random number generator.

I recommend using one of the following:


PBKDF2 with a single iteration. This is actually what scrypt uses internally to consume the input and produce the output.
HKDF - It's a very nice and simple design, standardized by NIST, has a RFC. Implementing it on top of HMAC-SHA-2 is trivial. It's my first choice as a KDF if the input has high entropy.