Buck has given you very good info but then he is a vet so should have that info. I had a jersey that had pre-calving milk fever every year, but she was an older cow. High producer.... easily treated. Had one that would get down after 12 hours or so after calving and we nearly always ran an IV and she would get going and be fine after that. I had one holstein get milk fever once but she was a problem cow in every way there was and I did not breed her back after multiple problems getting her bred the first time and the calving. Had an attitude too so that took care of that.
Normally, giving the bolus or the gel is enough for any that are borderline. Unless you are very good with an IV, and you know what you are doing, you can actually kill a cow giving calcium IV too fast.... I am not very good with IV except in the milk vein, and always use the gel and even some of the boluses....MUCH safer unless the cow is in dire straits.
Have never had that problem in first calf heifers but not saying it can't happen. Many of the dairies I milk test for keep the gel on hand to get them up and going fast.... and their "close up" pre fresh groups are monitored for the type of feed. Yes it seems counter intuitive to limit the calcium pre calving but that is what needs to be done.
My "poor jersey's" get neglected prior to calving in that they get no grain except a mouthful to keep them coming in the catch pens. Then when they calve, they will get grained and as soon as possible, I get them and their calves in, then keep the calves in and let the cows out so they want to come back in. Whether I make them into nurse cows, or I milk them, or do a "share" situation of the calf getting 12 hours and then locking the calf away, so that I can milk after a 12 hour stint away from the calf.... there are many ways to do it; they want to come to the barn and come in. Grain is also a great motivator. Some cows do better with share systems, some will hold their milk because they want their calf... Some will take any calf you want to graft on them, some are pure miserable b@#*ches and will only let their own calf nurse...every cow has it's own disposition. A nurse cow that will take any and all calves given to her is a true jewel.... read the thread by
@MurraysMutts on think she will make milk... his auction find that he named "Bessie Mae", has turned out to be worth her weight in gold.
I would not worry with keeping IV calcium on hand.... there are plenty of types of gels that will keep and are easier for a novice to use. If she needs IV, unless you are an experienced cowman, your best bet is to call a vet.