I'm attempting to represent a character who is a 12th level Wizard / 4th level fighter who then acquires 15 HD of outsider levels. Which base attack bonus is correct?
12th level Wizard / 4th level fighter (16 HD total) provides a +10 base attack bonus.
Base attack bonus can then progress in one of two ways adding an outsider's base attack bonus equal to total Hit Dice equivalent to a fighter:
+15 base attack bonus addition. Making a full attack at this base attack bonus is equal to +25 +20 +15 +10
17 HD to 20 HD provides a +4 addition to base attack bonus, this is accounting for 4 HD of the 15 HD outsider's hit dice. Making a full attack at this base attack bonus is equal +14 +9 +4 then following the rules for epic progression accounting for the remaining 11 HD of the 15 HD outsider's hit dice providing +6 epic attack bonus. Which in total results in making a full attack at base attack bonus + epic attack bonus of +20 +15 +10.
Normally, I wouldn't ask such a basic question. However, I note the Solar Angel is a 22 HD creature and its base attack bonus is +22; if it was following epic level progression, its base attack bonus should be +21.