Calculations based on approximates will be better to become a lot more precise, and one of these can vary considerably depending on the time of year.
There will be times of the year that are specific low flow times and those that are high flow. In my neck of the woods, we have two times of the year that are low flow, one is at the end of summer (when winter snowmelt on the mountains has been exhausted but before fall rains and cool weather increase the flow a bit again), and the other is about three quarters the way through winter (when most of the flow has frozen and not started yet to melt). This will be different depending on where you live. This will also vary slightly as to which week it is in a given year but knowing the basic time of year to calculate low flow is key to starting (as low-flow is about as close as can be measured to calculate the power that you can be pretty much guaranteed year-round).
Build a simple weir on the creek where (or at the basic height) you are putting your intake. This can be as simple digging a slot into each side of a small creek and cutting a piece of plywood to length to fit the width of the span of where you slotted in the banks. With such low flow, you can probably get away with cutting a notch in the plywood, so that all the flow goes over the notch, and then fill 5 gallon pales, or whatever, as your measuring device. A more precise measurement might happen if you were to fasten and seal a pipe in the plywood, and channelled that water into a container through the pipe. The less excess or splash the more accurate your measurement. Do a few trials to make sure of your accuracy, and once you have your method down, do three or four tries, three or four times during a couple of week period and make an average to figure out your guaranteed flow.
Find someone with a transit or other device to properly calculate your drop (Head).
Check out this
video, which is a fun
project done by a guy in Oregon who is playing with water coming off his roof into a rain gutter. He shows the way to calculate your power properly around 3:47.