You'll have to set it against a known pressure. The first time I used one, I pressurized a Unitank to about 5 psig and then backed the valve open until bubbles started in the liquid cup. When it came time to spund, I attached the valve, closed the blow-off ball valve at the bottom of the manifold and allowed pressure to build up until bubbles started. Then I'd close the spunding valve down slightly until bubbles started again. I kept doing this until the gauge pressure in the tank reached my desired pressure (15 psig). Then you can fine tune up or down and literally "dial in" your spunding pressure.
It sounds a bit more cumbersome setting it without any markings, but it isn't a big deal. The valves with markings are just approximate readings anyway. You'll still have to set final pressures with the gauge rather than etch marks. I haven't found any need to adjust the valve more than a quarter turn either way. Between uses I just leave it in the same position where it was. I suppose if you wanted you could mark your 'starting pressure' with a marking awl, but it's not really necessary.
Brooo Brother