Maybe you aren't satisfied with your computational skills. If you want to improve them, check the quality of your simple mental computations first. I keep in view simple mental addition and subtraction within the limits of 20, simple mental multiplication and division within the limits of 100. Everyone knows these operations must be carried out correctly and quickly. But how can we measure the correctness and quickness of their execution? For this purpose you can use simple tables which may be named stochastic.
You can draw such tables yourself, but do not waste time. Take the tables free at my site Prevention of Failure in School Math. There are two possibilities to get them: a Word file (TablesWord) and a computer program (PrintTables). The program allows to print both blank tables and checklists for them (the same tables but with the answers). The checklists will help you to find your errors. Furthermore you need a stopwatch to measure a running time.