Yes, this is a very good way to keep it consistent.
Likewise, If you take the mass of the solution and compare it to the mass of an equal volume of pure butane/propane, you’ll get a good idea of how much is in solution. Density of butane should be about 0.6 g/mL at (at either 0 Celcius or 25 Celcius, can’t recall)…if you have 200 mL of pure butane, it should weigh about 120g. Now weigh 200 mL of your solution with oil in it. The oil will weigh much closer to 1 g/mL. This can help you figure out the saturation consistently…maybe start pouring into graduated cylinders/beakers instead of jars, or mark the jars with sharpie yourself
Your jar does shows every 100 mL so you can still get a decent idea, but the more precisely it’s marked the better of course.