Mathematically, you'll lose 1/3 of the flavor. Subjectively, it's difficult to say how that will 'taste' to you if its reduced to 12mg strength. I've never tried the Dr. Pepper, and I've found some eliquids flavoring stronger than others.
If you find it's not flavored enough, you could do one of two things (that I know of):
1)
a) find a 0mg flavored liquid that is concomitant with Dr. Pepper and mix it at 2 parts 18mg with 1 part 0mg;
b) or find a 6mg flavored liquid and mix it at 1 part Dr. Pepper and 1 part 6mg. These combinations are just for example, and many other combinations are possible. There is a decent online recipe calculator here:
eJuice Recipe Calculator -- or an excel version:
http://www.e-cigarette-forum.com/fo...alculator-ms-office-excel-version-2003-a.html
2) learn about Lorann's flavoring's to supplement flavor to the reduction you need to make. I'm sure there are some flavors that will work well with Dr. Pepper, like cherry? Dr. Pepper always reminded of cherry coke, sort of. With this option, you could use a 6mg unflavored and supplement as necessary with your chosen flavoring.
I haven't yet used any Lorann's, but from what I've read on here, they do work. Good luck to you and the Dr.!