Thread-level Speculation (TLS) is a technique for automatic parallelization. The complexity of even prototype implementations, however, limits the ability to explore and compare the wide variety of possible design choices, and also makes understanding performance characteristics difficult. In this work we build a general analytical model of the method-level variant of TLS which we can use for determining program speedup under a wide range of TLS designs. Our approach is exhaustive, and using either simple brute force or more efficient dynamic programming implementations we are able to show how performance is strongly limited by program structure, as well as core choices in speculation design, irrespective of and complementary to the impact of data-dependencies. These results provide new, high-level insight into where and how thread-level speculation can and should be applied in order to produce practical speedup.