So, I finally convinced someone other than me to DM a game in my group. He's a big MtG fan, so he decided a Kamigawa-style campaign was in order. Cool beans, so I pointed him toward OA and the Rokugan third party materials.

In this campaign, I have decided to run a strictly non-optimized Shugenja 3/Bloodspeaker Cultist 10/?? 7. I say ?? for the last bit because I am unclear on mechanics behind Bloodspeaker's capstone ability, which states, and I quote:

Blood Path: At 10th level the cultist gains sorcerer as a favored class, and may continue to gain levels in the sorcerer class after he finishes this class. His spell progression continues from the last point that it left off.

Emphasis mine. You don't need sorcerer to get into the class, yet it specifically calls that class out, and does not define "last point". If I were to take the last 7 as sorcerer, would I be casting as a Lv 13 Shugenja /Lv 20 Sorcerer?
I understand the wording is really, really vague, and that its third party material, but before I get the DM to houserule I was wondering if this was exploitable.

*EDIT*
Bloodspeaker Cultist is from Legend of the Five Rings: Way of the Shugenja, p.91