Without having your part file (sketch) to work with, and just having my interpretation of your issue I think the solution maybe to use "Offset Entities" within your sketch. This will provide either a inner or outer (your choice) sketch of your current spline selection at the dimension you specify.
Yep, this is it. Sorry for asking a question that had such a simple solution - I often used Offset Entities Convert Entities (edited, I got the two mixed up) to project one sketch onto another plane, but didn't know it had that functionality.
And thank you, Wing. Your answer is correct, as well.