Yes, you learn more med math than is typically used in practice. Modern pumps with dose modes are miracles of nursing convenience, and of course are good as a safety measure
BUT, it is important to know *how to* do it. There could be a power outage and not enough red outlets, you could work for a medical mission someday, you could work in NICU or peds and have to do lots of wt-based calculations...not to mention PharmD's are human too. I once received a prefilled oral syringe of a med which based on the concentration and ordered dose, contained 2x the correct amount of med.
But you're correct -- lots of times the eMAR will specify the rate of infusion for an antibiotic.
Are you asking about atb's which are started prior to shift change but finish after? So say, a dose of Vanco started at 1845, shift change at 1900-1930, and Vanco finished at 2045? Let it finish. There is WAY too much room for error (plus wasteful) to calculate how much infused prior to shift change, and how much of a 2nd bag is needed. The day RN would document that she started the med at 1845, and you would document that the infusion was completed at 2045. Then make sure it's accurately documented on the I&O -- neither undocumented nor double documented.
If I see that the maintenance fluid is infusing and fluid still in the piggyback bag, I check the orders. Ideally if the MD had d/c'ed the med while a dose was infusing, the prior RN would discard the unused med. That would eliminate any confusion, and be the SAFEST thing if the drug was newly contraindicated (e.g. new evidence of nephrotoxicity, allergic reaction etc.)
If the pt was to receive it, and assuming the med has not yet expired, I'll finish that bag --
again to ensure that the pt gets the ordered dose of med. Sometimes a nurse can make a mistake when entering the VTBI, or even forget to open the roller clamp on the secondary tubing.