Just ran Castrum in full left side and right side SB gear. The gear was i49 HQ gear. From start to finish I got 65% spirit bond. My friend ran in with NQ equips and reported over 50% SB. It seems like since you have to do 2 runs anyway to fully SB, you could go into Cas with NQ gears instead of HQ. It'll save you quite a bit of money.