This app will save you time by doing the math for you.  
Want to know what plates to put on your barbell to go from 155 pounds to 235 pounds? No problem.
Need to know which plates to use to get 315 pounds with a 35 pound bar? Again, no problem.
In the middle of a power lifting cycle and want to calculate 35%-45%-55% of your working weight or max? Easy.

Note: Working weight is assumed to be 90% of your max.