When you're estimating testing you need to identify the scope of your testing - are we talking unit test, functional, UAT, interface, security, performance stress and volume?
If you're on a waterfall project you probably have some overhead tasks that are fairly constant. Allow time to prepare any planning documents, schedules and reports.
For a functional test phase (I'm a "system tester" so that's my main point of reference) don't forget to include planning! A test case often needs at least as much effort to extract from requirements / specs / user stories as it will take to execute. In addition you need to include some time for defect raising / retesting. For a larger team you'll need to factor in test management - scheduling, reporting, meetings.
Generally my estimates are based on the complexity of the features being delivered rather than a percentage of dev effort. However this does require access to at least a high-level set of instructions. Years of doing testing enables me to work out that a test of a particular complexity will take x hours of effort for preparation and execution. Some tests may require extra effort for data setup. Some tests may involve negotiating with external systems and have a duration far in excess of the effort required.
In the end, though, you need to review it in the context of the overall project. If your estimate is well above that for BA or Development then there may be something wrong with your underlying assumptions.
I know this is an old topic but it's something I'm revisiting at the moment and is of perennial interest to project managers.
