It seems like you’ve found something that works for your red grape winemaking — making additions based on your 36-lb. (16-kg) lugs of grapes. Your estimated yield for grapes fermented on the skins turning into wine seems about right. Most small-scale winemakers, often using hand-cranked equipment, can get about 155 gallons per ton (2,000 lbs.) or 590 L/910 kg of grapes, so your guesstimate of 5 gallons per 72 lbs. (19 L/33 kg) works out to be in that neighborhood. Don’t forget to adjust, of course, for any water you add.
I’m not surprised that you don’t get the best results doing the same types and rates of additions for whites. Though it might feel like you’re a little lost when trying to translate your red protocols to making white wine, the good news is that most winemakers find it’s much easier to calculate white juice/wine additions because you already know the amount of liquid you’re dealing with and dosing into.
Whites and reds are different animals. Whites ferment cooler and generally more slowly, which preserves the delicate aromas. Whites typically don’t need pectic enzymes unless your pressed juice had a high degree of solids when it was shipped to you. In my experience, they don’t need as many nutrients as a red fermentation because the grapes tend to be picked earlier so retain their nitrogen and amino acids better.
If you’re able to get Brix, titratable acidity (TA), pH, ammonia, and amino acids measured by a wine lab, those will be your best guides for where your white wines need to go and what kinds of additions you need to make.
For sulfur dioxide in white juice, I’m typically adding about 40 ppm (mg/L) at pressing. Be sure to check with your juice supplier on this; there may already be SO2 in the juice and you may not need to add any more.
With manufacturer-specific things like enzymes, nutrients, or even yeast, I definitely recommend reading the company’s literature and add conservatively in their recommended range. Most manufacturers have great websites with guides and their recommendations will most often be in easy-to-calculate measurements like g/L, mg/L, or mL/hL. If you need help doing any of those conversions, search the internet with conversion phrases like “L to gallons.” I often use the handy winemaking conversion and addition calculator at: www.winebusiness.com/calculator
To answer your last question: Yes, I would wait for a dry/not dropping specific gravity/Brix measurement as your signal to add the sulfites post-primary fermentation. Visual cap drop can be a sign for red ferments, but you’re correct, you won’t see something comparable on a white fermentation where the juice level in your tank or carboy will remain about the same.
