I would rotate with each injection site. I'm assuming you are talking about Penicillian shots. Penicillan can cause burning and soreness, so using a new site for each injection will lessen the soreness and the anticipation of your horse on where the owey is going to occur.

Is there a reason the vet didn't prescribe SMZ tablets (sulfa/trimethoprim)? These are just as effective for most situations where penicillan is used to treat infections. They are given orally and can soaked in feed/bran mash or placed in a large syringe with water to dissolve and given orally like paste wormer.

I don't know where you are located, but 2 different vet hospitals have diagnosed different horses at my barn in SE VA with Potomac Horse Fever.  One current horse, they have said has PHF or Salmonella.  None of the horses have been significantly sick (my vet and another vet, not involved with any of the horses, YET, both aren't buying the PHF diagnosis too much because of the lack of sickness).  First horse had two swollen back legs and slight fever.  No diarrhea or other signs of illness.  Immediately put on IV Oxytetracycline.  Seemed to be fine, but developed laminitis a couple days later.  Now, a couple weeks later, same horse ran major fever (104.1) and vet said it is Flu, due to impaired immune system from PHF.  Whole barn is quarantined for 3 weeks.  Two other horses (mine included) had swollen back legs, about two weeks ago, but no fever, before the "Flu" episode.  My vet said it was most likely an immune response from fighting off a virus, posssibly something she was vaccinated for.  He prescribed hosing and wrapping legs and giving 1/2 dose Banamine twice a day.  I did this for 2 1/2 days and all is fine.  Other vet says my horse and other that was even less effected had PHF.  Just Sat., another horse had mild diarhhea and highest temp of 101.9 and one swollen back leg and a third vet said PHF or Salmonella.  He hasn't been sick or off food or had terrible diarrhea, just soft, cow like feces.  They took a sample to the State Vet lab, yesterday for testing.  They've done nothing to treat him, so far.  I'm beginning to wonder if on the way to the barn, the vets aren't just opening up their Merck Manual and picking something on the page!  I've had horses for almost 30 years, many of those in large, active show barns and never seen any of these things.  Anyway, because of recent floodings, I've heard lots of other folks talking about PHF in their areas (Kentucky, Missouri) where it hasn't usually been seen.

As far as your injection issue, 28 cc seems like an awful lot on Pennicillin.  I think I'd split each injection into 2 spots.  I know, it means more needle sticks, but 28 cc in one spot can cause a lot of discomfort.  Hope your guy gets better quickly.  I know we are at about our wits end at my barn.  It's been there for about 4 years and we haven't really had any sick horses at all and now possibly three major diseases strike in a month?!  All horses there are well vaccinated, daily wormed, and generally very healthy.  Go figure!