MarkT:
Rds(on) doesn't change much with current if the device if Vgs >> Vds and you can model it as a simple resistance.
You can't calculate Vds(on) and know that Vgs >> Vds until you know that you can model the mosfet as a simple resistance. So your argument is circular.
Data sheets always quote Rds(on) at a particular drain current. Where Rds(on) is quoted for two different values of Vgs, Rds(on) at the lower Vgs is typically quoted at a lower drain current - often much lower than the current rating of the device. The reason is that at lower Vgs, the drain current at which the mosfet transitions from resistance mode to transconductance mode is lower.
By ensuring that the current you are switching is not greater than the value at which Rds(on) is quoted for the Vgs you are using, you ensure that the mosfet is fully turned on for that drain current and the Rds(on) figure is valid to use in your calculations.