Yes, you can use an actual-business percentage.

However, I usually don't accept random numbers.  I want them to have some sort of solid, logical reason or documentation if they want to use an actual-use percentage rather then the Time/Space percentage.  For example, saying 50% of utilities seems completely random unless they have documented usage from before they started daycare (and each utility should be assessed separately; just because their electric usage doubled doesn't mean their water usage doubled).

Landscaping is not 100% even if they only did it because they have daycare.  They still personally benefit from it, so it is only partial business use, such as the Time/Space percentage (personally, I think using the "Time" percentage is more logical because the space percentage doesn't really affect landscape usage percentage).

Don't forget to increase the Time/Space percentage by adjusting things for the exclusive-use bathroom.