Hey,
These two documents show that the RL10 CECE Base achieved a 17.6:1 throttling ratio.
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20100032918.pdf
http://www.rocket.com/files/aerojet/documents/Capabilities/PDFs/RL10 data sheet Feb 2016.pdf
Perhaps that would be a better value instead of the current 15:1?