Use timed road testing to set advance. Stop when you are at best time for the runs which reflect your desired driving pattern. To reduce variability, use Wide Open Throttle wherever possible, make your runs on level ground, and do multiple runs in both directions. Please do all this in an open area with good visibility, and slight or no traffic. The road which runs along the river or coastline is a good bet. Consider that you want your average speed over the course to be similar to or slightly below the speed limit. That will attract the least attention and tie up the least traffic. Note: This consideration is why 0-60 testing on open roads is NOT recommended. Pick a narrower speed range.
Use a STOPWATCH, not a regular watch. You'll need to record times to about 0.1 seconds. Use a buddy to time if at all possible while you concentrate on driving in a consistent manner over the same ground, and looking for other traffic! Try to run all your timed runs in the same basic temperature conditions (10F temperature degree change is worth about 2% change in air density, and so mixture). Also, try to maintain a similar weight in the car (with or without a timing partner, full gas tank vs nearly empty, bags of cement or cases of Old Clem's Joy Juice in the trunk, etc).
If you are shooting for best mileage, your tests should consist of sustained runs at the speed and over the terrain you plan to use. Testing with a normal mix of highway and around town can be misleading since your mind-set affects your gas pedal usage. Changes of less than 2 or 3 mpg in a normal mix can be ascribed to psychology as much as physics. So, you should fill the tank, drive at least 1/4 of a tank at sustained speeds, and then check the mileage.
Now you should have a set of benchmark results for further use. Open up the access port in the top of your carb, and remove and inspect the main jet assembly. The part/size numbers are stamped on the side of each component. Record all numbers.