Blegh, so we've had our starting reputation with Guilory and Daurstein ruined? Damn it all...
So, I set the unit up with 200 Trained, 350 Regular, 450 Professional.
If we take Trained at the Base, then 350 Regulars push them 35% of the way from Trained to Regular, or 3.5 XP. Meanwhile, 450 Professionals then add 45% progress from Trained to Professional, which is 13.5 XP.
That's a total of 17 XP, taking the 10 needed to promote to Regular and setting them at 7/20 of what's needed for Professional.
If we use Regular as the base, then 45% of the way from Regular to Professional is 9 XP, subtracting 20% of the way from Regular to Trained is -2 XP, for 7/20 XP, or the same results.