You don't need an accurate measurement as there is an operating range for the oil level that proves satisfactory. If you check the oil with cold engine, all oil should have drained down and so the oil should read as high as it will ever read without adding more oil. This oil level should always be maintained at the full mark even if you have to add small amounts to get it there. I usually have a quart bottle handy in the garage to keep the oil level serviced.
There's no need to check your oil at every gas stop. If the engine is in good condition then this is a waste of time and risks contamination of the oil. If the engine is new and broken in, check it every 2000 miles else check it every 1000 miles. In newer engines the oil should make 2000 miles easily before the level drops to the add mark. In older engines or loosely built engines check it at 1000 miles. If you're not comfortable with this, check it more frequently but check it in the morning before you use the car rather than wait until you arrive at the gas pump. Changing oil at 4000 miles is a good practice for long engine life. You can change it earlier but you're wasting money for all modern SAE oils used in engines in good condition will wear for 8000 miles before starting to break down. Always change your oil not greater than 8000 mile intervals.
There's no need to check your oil at every gas stop. If the engine is in good condition then this is a waste of time and risks contamination of the oil. If the engine is new and broken in, check it every 2000 miles else check it every 1000 miles. In newer engines the oil should make 2000 miles easily before the level drops to the add mark. In older engines or loosely built engines check it at 1000 miles. If you're not comfortable with this, check it more frequently but check it in the morning before you use the car rather than wait until you arrive at the gas pump. Changing oil at 4000 miles is a good practice for long engine life. You can change it earlier but you're wasting money for all modern SAE oils used in engines in good condition will wear for 8000 miles before starting to break down. Always change your oil not greater than 8000 mile intervals.