Not necessarily a "guide", per say, but here is a handy calculator that will let you know how many silverhawk feathers you will need to get from your current XP to your desired level.
Silverhawk CalculatorTarget Level: Enter your target level here, from 2-120
Current Experience: Enter in your current experience, or the experience at the level that you'll be starting your silverhawk-specific training.
Bonus XP: Enter in your amount of bonus XP in agility - this will be factored into your necessary number of silverhawks.
Don't forget to press submit.Here is an example of what it should look like.
If you would like to find out the approximate cost of said silverhawk feathers, go to this link:
Silverhawk Feathers WikiYou should enter in the amount of feathers you need in the "exchange" box, as shown with my example from before.
Keep in mind that the buy limit is 250 per 4 hours, so it would take about 6 days to buy this amount of silverhawk feathers.
Happy (buyable) agility!