I use conversion factors or scales in situations like these.
I would not recommend making up a water solution of Rennet as it will likely degrade over time, as you suggest. In home cheesemaking that could be over a long time, unlike professional cheesemaking where this might be a week's time and cause no problem.
Better to use a small ( maybe 5 gram total) electronic digital scale on the market ( try e-Bay if a regular search doesn't produce results). They are very inexpensive and are good to about 0.1 grams. Very portable about 3 " X 5" or so and they close up like a book.
I am told they are used in the illicit drug trade, but they also have many legitimate uses and home cheesemaking and winemaking and beermaking are three. They can even be "tared". This means a small piece of wax paper can be placed on the scale table and when the scale is turned on, it will read zero or there may be a button which tares the paper. Now add the ingredient to the desired weight read directly from the digital meter on the scale. Remove the paper and material weighed and add the material to the cheesemaking "Vat". Repeat as needed. Useful also for potassium metabisulfite in fermentations.
If volume, rather than weight, is to be used, a standard ( medical) teaspoon is 5 grams or milliliters of <water>. This cannot be used, without bulk density conversion, for other substances like salt ( rennet) or LYO ( freeze dried) starters.
Bulk density is just how many grams are in a milliliter of volume.
Salt has a bulk density of around 2g/ml, so a 5 mil tsp of salt will weigh 2g/ml X 5g/tsp or ~10 grams/tsp. I do not have the bulk density of the starters, but would guess about 0.5, so 1 tsp of starter would perhaps weigh 0.5g/ml X 5g/tsp or 2.5 grams.
Likely, you have these bulk densities for your products so can create a conversion table such that 1 tsp of Choozit weighs 2.5 grams or such, 1/8 tsp etc.
Another method with dry powders: use a sterile piece of paper and put a known amount ( say 1/8 tsp) onto the paper in a symmetrical flattened pile. With a sterile knife blade ( flamed and cooled) divide the pile in half and separate, quarters, etc. SO that if 1/2 tsp of Choozit weighs 1.25 grams you can easily measure out 1.25/4 = 0.31 grams or 1.25/3 = 0.42 grams or 1.25/8 = 0.15 grams. The results will be approximate but good enough.
Real danger is contamination since the unused portion will be returned or placed in a separate envelope and possibly contaminated. The risk in this case is small since the substance is returned to a freezer.
Also 2 Tablespoons of water is about an ounce of water. 1 Tbl is 1/2 ounce. 3 tsp is one tablespoon so 1 tsp of water is 1/2X1//3 or 1/6 of an ounce and also 5 grams.
So for water:
1/2 tsp is 1/2X1/6 = 1/12 ounce or 2.5 grams
1/4 tsp is 1/4X1/6 = 1/24 ounce or 1.25 grams
1/8 tsp ( half of 1/4) 1/8X1/6 =1/48 ounce or 0.625 grams
20 drops from an eyedropper is 1 ml of water so 10 drops is 0.5 ml and so on. This number is pretty independent of the eyedropper as long as the drop stays on the end of the dropper - it is related to the surface tension of the fluid ( in this case water).
Hope this is helpful.
Although store bought, pasteurized, homogneized milk is vilified in all amateur cheesemaking books, I have never had a batch using this kind if milk from various dairies, to fail. Also, I keep seeing "add calcium chloride". The amount is so small ( 1/8 tsp) that it is like chicken soup as a medicine. It can't hurt, but probably doesn't do anything. Calcium chloride used to be necessary during the WWII since they were using hog based pepsins and such.