I'll probably end up mixing 70% synthetic with 30% of my own just to ensure it passes in the lab. Even without dilution, I'd give myself an 80% chance of passing. Basically I'll be diluting with synthetic urine.
Mixing your own urine with synthetic is an unwise move, especially if your urine is diluted and/or not completely drug free. And if your own urine is indeed clean, then why bother with subbing?

I'm very close to being completely clean as is.
How do you know for sure that youre "close to being completely clean" without having a quantitative analysis performed on the urine sample? Home tests (as well as the initial assay screens performed at the lab) are a "go/no-go" endeavor and lack the ability to provide a quantitative value. In order to obtain quantitative values, the urine sample would have to go thru the GC/MS.