Generally speaking the dynamo/battery/lights combination is chosen to more or less balance under average driving conditions but there is usually just a bit in reserve.
With a good battery and original wattage bulbs and at a speed of around 30 mph I would expect to see zero or a very small charge on the ammeter. This is because what the ammeter is actually showing is the balance between the dynamo output and the current draw being demanded.
If the battery has been partially discharged for any reason, due say to the lights being on without the engine running, then I would expect to see a +ve reading on the ammeter with the engine running even with the lights on. This charging rate should never be more than an amp or two if the regulator is doing its job properly, that is a part of its function, to protect the battery from being overcharged as well as adjusting the dynamo output to balance the load. You should also expect to see the charge rate tail off as the battery is topped up.
It would seem therefore, from what you have said, that your system is working as intended.
