"Over-exertion" is not necessarily based on the number of miles in a given day. Typically it is measured by anaerobic efforts (90%+ of max HR), but as long as you don't bonk, it doesn't matter much how many miles you're riding aerobically. Too many miles in a day may result in overuse, but determining what mileage will result in an overuse injury via mathematics is effectively impossible.
I don't think it makes much sense to try and make any sort of online calculator for this particular task. There are simply too many factors and variables that almost all tourists don't bother to quantify or measure. E.g. it's exceedingly rare for tourers to use HRM's, and even rarer to use a power meter, while on tour -- let alone anything to accurately measure wind. I can't imagine anyone other than a fanatic who writes for Bicycle Quarterly booking time at a wind tunnel to quantify the aerodynamic penalties of their touring setup. Your weight (gear and body), the route's elevation, temperature (yes that can matter), weather, your start and stopping points, distances between services, time taken to eat etc is going to vary on a daily basis. Of equal importance is mental aptitude, which cannot be mathematically analyzed in any meaningful fashion.
The primary benefit of the Kreuzotter-style calculators, despite the specificity of the numbers, is in painting broad strokes of the effect of a given variable on performance. So you can figure out that small changes in weight have a minimal effect, or that a wind needs to be fairly stiff to have an effect, or that the time you gain on a descent is very small compared to what you lose with the climb, the general effect of using skinny high-pressure tires vs treaded tires, and so forth.
It's much better to do a few tours, figure out your approximate speed including stops, and use that as a guideline for how far to go in a given day. A good baseline will and a little experience will provide much better results than the illusory precision of a mathematical formula....