Thank you jmdoty for your feedback. The issue of rounding is something that has been discussed before. The reason the calculator does not follow the exact same approach as Bradley is because the guidelines to not tell the user they have to do it that way. You've quoted the guidelines in your post, and since it says "may be necessary" rather than "should" or "must" the author has decided to use the same approach as looking up values from the Federal 1040 tax forms. This approach uses the lower value until the threshold is crossed.
If you think about this approach, it does the same thing, spans the same income range, and is less confusing. For example, if the income in the tables is $3000, $3100, and $3200, the income span between the values is $100. From what you are saying, the span should really be $2950-$3049, $3050-3149, and $3150-$3249. So, the income spans are the same ($100), but depending on the method, a different child support amount can be seen. Personally, I think our approach is easier and makes more sense.
Another user made the comment that he preferred to use the logarithmic equations rather than the tables. This is just another approach, but the guidelines say that for income beyond the tables to use the log equations, so the calculator here uses the tables until income beyond $15,500 is exceeded.
So, the approach of our calculator doesn't aim to follow anything done by Bradley, but rather the guidelines themselves. When something like rounding is left to the user's discretion, ours apparently takes a slightly different approach. I'm glad you were able to modify the calculator to suit your needs.