[GpsFrontier]

As the plant uses nutes it may use more of one mineral and less of another so the more you have in the res the better you are.

Yes this is the reason that more is better in general. I read somewhere that strawberry's need a minimum of .5 litters of nutrient solution per plant. I don't know if this is true, and I have never found a chart with requirements for this information to compare with for any plant. Some plants are heavy feeders and should use more nutrient solution because they use the nutrients faster than plants that are not. Also how often you change the nutrient solution is a factor.

There are also other things to take into consideration like how big the plant is (seedling/full grown), air temp, type of plants etc.. The type of plant can play a big difference in how much water it drinks. A full grown strawberry plant wont drink as much water as a full grown broccoli plant will. The air temp and humidly affect how fast it will evaporate also. Amigatec's suggestion is a great place to start, although careful observation is important also. I would suggest marking the inside of the reservoir with a permanent marker at a known level, or in increments to be able to keep track.

Also when adding water to bring it back up to the right level, don't add full strength nutrient solution. Either just plain water or a light diluted nutrient solution should be added. There is no formula to figure out how much to dilute it that I know of. Some people use a PPM or TDS meter to judge how strong there nutrient solution is and make adjustments by its readings. I personally don't have one. I change my nutrient solution every one or two weeks. I never add any diluted solution the first week but I will probably add some the second week, and depending on how much water I have needed to add during the first week I will decide how much I need to add for the second.

Would that be due to a plant's particular needs or for any number of reasons ?

Basically, not all plants use the same nutrients in the same quantities. Even the same exact plant side by side might use slightly more of one than another. The stage of growth they are in also plays a factor in what they need/use.

My plan is too use distilled water , store bought. I'm not going to deal with our tap water.

I wouldn't use distilled water. There are micro elements in water that are necessary for the plants. They take this into consideration when they make the Hydroponic nutrients, so they don't have too much of these micro elements (unless they are made for pure water). If your tap water is bad you may find it best in the long run to invest in a inline water filtration system. You could probably find one for under $100.