There is no formula for calculating the crossover speed, but fortunately the TP’s at Boeing have experimented and found them for you. “Them” because it is not a single speed it depends upon configuration, loading etc. They are in the range of 170-190Kts so don’t worry about calcs with Mach or FF!
The speeds are in the range were you are using flap 2 to 10, which is why back in 2000 the block speeds for these settings were increased by 10kts to increase your margins (and also why flap 2 disappeared from use). If you fly at the standard flap speed schedule it will never be an issue. If you find yourself slower than the flap speed schedule and with full rudder applied, you really need to accelerate asap otherwise you may lose directional control.
Description & background available at
http://www.b737.org.uk/rudder.htm
S&L