Yes, we do have good online
soap calculators available in recent years, but it hasn't been all that long ago that Soapcalc and the other oldest of the online calcs simply didn't exist. The internet wasn't available to most people in the US until the early-to-mid 1990s. The earliest archive on the Wayback Machine of Soapcalc is from 2009, although Soapcalc might be older than that.
Before then, one did the calculations by hand and the results were just as prone to the same types of errors and variability as the results from online calcs.
Certainly the online calcs are good enough for the average to experienced soap maker to get consistent results when making liquid soap as well as bar soap. Bar soap, being a solid material, hides a lot of sins, and liquid soap is less tolerant of inaccuracies.
Those inaccuracies are there whether one makes liquid or bar soap, however. It's just that problems are easier to see in liquid soap and easier to blissfully ignore in bar soap.
You can't go by what you see in the forums on SMF as being representative of the entire liquid soap makers out there. We generally hear from the few are in trouble, not the majority who are successful. I think there are a lot of people making liquid soap and having few or no problems.
Here's something you can do -- Open your preferred soap recipe calc and set up a liquid soap recipe using Susie's parameters, then make the soap and evaluate the results.
Is the paste zappy even after a few days and even if you're sure you made the soap correctly? In other words, is the recipe lye heavy in a way that can be traced back to the recipe calculations?
Do you see any white fatty layer on the diluted soap that indicates the soap is too fat heavy even if you've made the soap correctly? In other words is the excess superfat something you think you can trace back to the recipe calculations?
If you don't see either of those issues, then the calc does a good enough job to pass muster.
Even if you do have excess fat or excess alkali, those problems can often be corrected in liquid soap, although it's certainly easier to get it right the first time than to fix problems.