Square root times nine is a rough guide, the best way to avoid aquaplaning is to hammer the sucker onto the runway to punch through the standing water, {Ive never had a problem doing this!} However, once you do start to plane it may stay in this condition to speeds way below the suposed magic number. The Viscount with its very soft long stroke gear was very prone to this, even our little 421 finished up with rubber reversion recentley after failing to get wheel spin up on touchdown.Manual spoilers, lots of reverse, and a firm landing will do much to avoid the problem.As for touch down speed, slower is better, but dont create another problem by floating down the runway trying to kill speed.