I think your method is just fine!
There are a couple of schools of thought on the amount of Prime to use. Some say use an amount for the entire tank volume regardless of the volume of the water change (after all it's a mere capful for 50 gallons!). Others say just use the amount for the new water being added. Since Prime can't really be overdosed, the safe method when directly filling the tank from the faucet is exactly what you are doing by adding the proper amount of Prime for the entire tank, so it's effectiveness is not diluted, then add water with hose or Python like device.
Of course, if you were filling with buckets, you'd only add the amount of Prime required for the new water being added (2 drops per gallon).
As to the volume of the weekly water change, this is another great debate. Many experts recommend 20-25% weekly. Some here feel 50% weekly is required.
Frankly the required volume of a weekly water change is really relative to the bio-load, tank maintenance and water filtration/purification.
In general, 25%-50% is a good idea, with more being slightly better than less. In my case, I have very (too) high nitrates in my well water so I have to special filter water (or buy it!) for water changes. Using good tank maintenance and advanced water filtration/purification, I am changing 10g - 15g weekly water changes (usually 10g) on my 60g tank. This is 15% - 25%. I have been doing this for three months now with very good success...knock wood