Be careful with white teas. White teas can have as much caffeine (sometimes more!) thank black tea.

I was able to find some information about relative caffeine levels from a lobbying organization, so take with a grain of salt, but according to these folks Lipton decaffeinated green has less caffeine than a 16oz Starbucks hot chocolate. I can't find any information on their methods or where their data came from though. This study indicates that starting from a fully caffeinated green tea bag, you can get down to 5mg/6oz with successive steepings, which is about on par with what the FDA says is contained in hot cocoa. This study was not peer reviewed that I can tell; its findings indicate slightly more caffeine than the previous one I linked (note: their brew cycle was three minutes, not five). It also claims that tea made from twigs has less caffeine than that made from leaves, a finding I can anecdotally confirm as I experienced terrible caffeine withdrawal symptoms when I tried to switch from oocha to kukicha a few years back. I would assume that commercial decaffeination processes are more effective than the home washing method, but I can't find any studies to quantify how much better they are.

I only keep using hot cocoa as a reference point because I've seen you drink it with no ill effects, but I feel like I ought to point out that there are other alkaloids in both cocoa and tea that might have an interaction with the caffeine to make your response better or worse, respectively. Please use caution if you decide to try this experiment on yourself!