Why are you trying to split tax? It might help to be more specific about your question.
Are you trying to prorate your tax across multiple category splits or are you trying to separate out tax into a separate category of its own?
For the former, I use a calculator. For the latter, well, I don't think that's very useful.
Okay. I will spell it out. I buy 2 items for a total of $21.10. Each item cost $10. The tax is $1.20. Do I have to manually split the tax up between both items and add it to each item? I just figured the software would take the tax amount and split it between the transactions. An I missing something here?
YNAB doesn't currently automatically split tax. It's up to you to figure out the splits, either by using a calculator or rough approximation (what I usually do). I don't particularly care about exact cents. As long as I'm roughly within a dollar, that's really all I care about.
If you mostly use the desktop version of YNAB and are a Chrome user, the next version of the Toolkit for YNAB (http://toolkitforynab.com/ - a free Chrome browser extension that adds tweaks and minor new functionality to YNAB) includes a button that can automatically split the remainder amount between your splits. I'm not 100% sure when the next version will be coming out, but I believe we're roughly scheduled for another release this week?