Nothing stopping you from computing an average day and picking the random
numbers in that range. Granted, if it's a really slow day and your random
is skewed towards the high end you may not get your 1/10th. And if
they're skewed towards the low end you may end up doing 1/5th.
And if the line blows the doors off average recalc with a spread going
from average day to record day.
But, on average for the month, you should be hitting 1/10. Unless, of
course, each new day is a record sales day. God grant you such problems!