(You can get 6 missiles into the glass before killing the Rinkas, but 4-5 is quite a bit safer)
If you have Space Jump this is the best strategy to set up the rinkas and do away with Mother Brain 1 quickly. It saves at least a second over the strategies without Space Jump that you would see featured in PRKD any% runs. it is a little bit risky, however, because depending on how you fail it the Rinka positions could become absolutely awful and cause major time loss.