I use the inside of bend to inside of bend. I don't think of bar rate as an accurate number, just as a way to compare one bar to another. There are too many unknowns, like bar mount flex, end link flex, frame flex. Then the bar has a motion ratio that translates to wheel rate, so the bar rate at the wheel is maybe half.
changing from splined bar to splined bar is easier to compare. The Global West and DSE numbers might be inch pounds per degree or per 5 degrees, hard to say.
I can tell you I have used the Hotchkis hollow and it's about right, the Hellwig is one step stiffer but it will take more rear bar to get the car balanced assuming a front tire if 275 & rear if 335. If tires are more equal the Hellwig works OK.
Try using this HTML sway bar calculator. The original web calculator is no longer active but I saved a copy of it and it works pretty well. Great for comparisons. I would reload the page when entering new bar dimensions, it sometimes can give wrong results if you jump in and change the numbers, but I'm on a Mac with Firefox, so maybe it's my computer.
Note keep the slider for motion ratio to the right at 1 to 1 unless you know your sway bar motion ratio.
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This is a free download hosting site, so it will have popups, sorry.