Total tax rate is more of an estimate. If you're buying new, the sales agents should be able to give you the base tax rate plus the dollar amount of the Mello-Roos. When we look at VoC, the Mello-Roos amounts were fixed dollars based on sq footage of the home with the dividing line around 2,500 sq ft. Under 2,500 you paid one amount, and if over 2,500, you paid a higher amount. If the sales agent can't get you the amount, then call the City. For VoC, I think it would be the Redevelopment Department. They will likely give you the number of the financial advisor for the bonds, who should have the breakdown.
For re-sale properties first occupied from June 2006 and back, check the OC Tax Collector's site and get the property tax detail.
Note that there are fees above and beyond the base tax and Mello-Roos on your property tax bill. There will be a Vector Control district charge (~$10), possibly a Metropolitan Water District charge (~$100), and perhaps some others.