This is a very good question. Even I had this same thought when coming up with the formula, So I had to test it out. I took a costumed Rigard (5% mana bonus) and a level 1 mana troop (5%) thus giving us a total of 10% mana boost (MB = 10%). I ran numerous simulations and one particular case helped answer this question.
I made a purple dragon bomb and then burst that bomb, thus giving us a total of 5 purple tiles. Now after Telluria fired I immediately made 2 x purple 3-matches (total of 6 tiles) which charged up Rigard fully. Now this was very interesting to me. Because depending on which method we use - additive or multiplicative - the result would have been different.
From the formula given in the post the total mana generated in this scenario would be:
5 x (110) + 6 x (110 - 34) = 1006 ( just enough to fill Rigard’s 1000 capacity mana tank)
But, if we consider Telluria’s effect to be multiplicative it would have been:
5 x (110) + 6 x (110) x 0.66 = 985.6 (just shy of filling Rigard’s 1000 capacity mana tank)
So, from there I concluded that SG applies the mana debuffs on the bass value and not the modified boosted value. Hope this helps.