To calculate fluids you take the weight in kg and you can divide by 20 or multiply by .05. In your case,formulating would go for about 85ml of total hydration per day. I think what your vet is doing is a support dosing.....and I feel the amount your vet is suggesting is appropriate. If your rabbit is on his side, that makes him prone to Pulmonary Edema so you would want to closely manage the fluids to prevent fluid from building in the chest. That can walk you into more problems. And remember that any water he actually drinks as well as fluids contained in greens also count toward the total hydration. Keep in mind that this calculatedamount is the baseline we use for animals coming into rescue. Actual amount of fluids is determined by the hydration level of the animal. That is best measured by the amount of urine produced. Judging by your comments about your rabbit...I would stick with your vet's instructions.
Couple of things with injections....don't stick them any more than you have to, use the smallest gauge needle possibleand combine meds when possible. In the case of Penicillin.....buffering with fluids helps with two things. First, refrigerated Penicillin is very thick so warming up the dose and putting it in the fluids will make it easier to inject. Next, Penicillin burns like H-E-double hockey sticks and buffering will cushion the burn. And many injectables can cause necrosis at the injection site. Penicillin is one along with Baytril (and most of you know I do not like Baytril). I always try to do injections once a day if at all possible. In this case, load a couple of syringes. Do it all at once. I have had a line of syringes loaded with fluids and meds and administered them all with one punch. You also need to move the injection sites so as to not cause scar tissue. Any place you can scrff skin you can sub-q. I start high on the shoulders on either side of the spine and moving down, varying sides of the spine and including the flanks.
I am thinking your rabbitis being treated foran ear infection. But the fact he is on his side really concerns me. Just wondering, does his left rear leg seem to have anydifficulty? Does he have nystagmus (darting eyes)? What is his rectal temp? Has a blood chemistry been performed? I think something more is going on here. I would be really interested in more details since head tilt is one of "my things" and I might can offer some information that very few vets have regarding tilt. If you are interested, please PM me.
Randy