I really like SoapCalc for the brevity of the first few lines, and how it can handle changing from percentages to weights and back (it makes it very easy to resize in SoapCalc - something that Soapee is not great at).
Mixed liquid ingredients:
I would like to see an adjustment to the water amount for mixed liquids, that contain either fat or acids in liquids, so that the water content is also taken into account as well as the the required hydroxide needed.
This feature would be very useful for a number of liquids (like vinegar, goat's milk, cream, coconut milk etc.).
Having default percentages (of fat or acid), but being able to manually adjust the percentage on these ingredients would be useful.
For example, some people have 5% vinegar as their usual strength, but other people buy 6% vinegar strength.
Another example is milk - it could be a default skim milk 3%, with the ability to enter 6% for the fat content instead.
Master Batch Lye:
With the increase in split lye (for milk soaps) and masterbatching, I think it would be an excellent selling point to have a
lye calculator that can handle master batch liquids (currently all lye calculators require people who master batch (or use the split method) to manually calculate the adjustments to both lye and water (which increases the chance of error).
This could be achieved by adding something like a tickbox for masterbatch/split, and an entry box for the lye concentration percentage (with a default to 50%).
The output would be the weight of the masterbatch solution and the weight of the addition liquid required (to bring it up to the desired lye concentration/ratio).
Unit of measurement:
If possible, I would like to be able to print out my recipe only with my chosen unit of measurement (to reduce the chance of error due to reading from the wrong column).
Dual lye:
I'm another one who would like to see dual lye on a SoapCalc style calculator, with a percentage of each hydroxide being manually entered.
Hydroxide purity:
I would also like to be able to enter the purity percentage for sodium hydroxide as well as potassium hydroxide.