Hills say
All visible within 2BW over a ridge
Uphill Combat Claim if entirely on slope and nearer centre/ridge than opponent
So if just behind the ridge I can only be seen at 2BW
Therefore a charge can only be declared if the enemy are within 2BW
Assuming the enemy charge from 2BW, I can counter charge cross the ridge and hence claim uphill combat.
In MeG you do not have to be able to see your target to charge them (maybe a bit odd, but there you go), so as long as a target is within reach across a ridgeline (or in visibility restricting terrain) you can charge.
In your example, if you are charged from 2BW away you could indeed countercharge to a position 1 BW away from the chargers (as this is the closest a countercharge or intercept can go to an enemy charging them) which would put you on the ridgeline.