Fresh out of residency and at my new job I'm the first person to do these blocks at my place. Surgeons don't mind me doing them and are kind of intrigued, especially the younger ones. Here's the routine I typically do:
If performing bilateral mastectomy, intubate patient with 100 mcg fentanyl then perform bilateral pecs II blocks using 10 mL for superficial, then 20 or 25 mL for the deep injection. Use 0.2% ropi with 2.5 mcg/mL epinephrine and this should total about 1.5 mg/mL (toxic dose 3 mg/mL with epi). This should give enough room for surgeon to give local too. After this, give 1 mg hydromorphone up front and that should be enough opioid for the whole case.
If unilateral mastectomy, intubate patient with 100 mcg fentanyl then perform a pecs II block and consider using 0.5% ropi with 2.5 mcg/mL. With axilla dissection, consider supplementing with intercostobrachial block.
I've played around with using (SAB + PECS1) vs PECS2. The SAB doesn't really seem to make a big difference IMHO, plus it requires 2 more needle pokes for bilateral breasts.
My technique is to prep all 4 needle poke spots (PECS1 sites and mid-axillary line SAB sites) with arms out at 90d, I then do both the PECS1's then reach over further and do the SABs, again mid axillary line in plane with needle tip angling cephalad entering skin at about the 5th or 6th rib targeting the plane at about the 4th rib.