The best fuel economy is achieved as the maximum vacuum level that you can achieve under the conditions at the time.

If you can hook up a vacuum gauge, even if only temporary, you can test drive over roads that you normally use & try different speeds to give you different readings for comparison.

If you can find a good flat road somewhere then try driving up to whatever speed that you want, shen slightly reduce the throttle.
If you are careful you can cut the throttle back & raise the vacuum level without affecting the road speed.

Gently does it & you will be surprised just how little throttle opening it takes to keep your Datsun rolling along at a reasonable speed.