You just need to be able to figure the system volume and then it's 1 gm of salt for every 1000 mls of system volume. You'd just weigh the correct number of gms of salt dissolve it in some of the system water and then add that slowly to the system. If you do water changes then you'd need to figure how much water you replaced and add a bit more salt to compensate. If it's due to evaporation then you wouldn't need to add any.
The veggies you've listed should be fine with the low salt concentration. I have noticed that celery actually takes on a salty taste at higher concentrations.
If your kit is maxed out you can dilute the original sample, say one part sample to 3 parts tap water and run the test. When you've got the result multiply it by four (assuming there are any numbers on the test at all
). If you want a better kit then the API Freshwater Master Test kit is what many people here use - it's not perfect but does well enough and may be better than what you have.