To my knowledge, there are 2 ways to do your expenses. The formula (days open/total days in week x hours open/total hours in day x rooms used for dc/total rooms in house) OR if you have half of your space dedicated to daycare care use only. I have exactly half my house for daycare only (basement) so I am allowed to claim most expenses at 50%.
The first few years I did my groceries separate though and claimed 100%. It wasn't totally accurate however as I would usually buy the meat on my personal bill and I didn't claim a lot of meals since they were from our previous night's supper and I didn't know how to figure that out lol. I now buy my groceries all together and claim 50%. It's still not a true number since they don't eat steak and lobster like we do haha. (Altho, they eat a ton more fruit and veggies than we do so maybe it evens itself out over time?) However, it does work out to being more to claim which is what my accountant suggested to do. It's legal even though it's not completely accurate. It's no different than claiming my other bills 50% even though they aren't watching my tv or using my lights half the time either.