Your math is wrong yeah
Using the 50 per reward I used above:
Reward 1 - 50 Valor
Reward 2 - 100 Valor
Reward 3 - 150 Valor
Reward 4 - 200 Valor
Reward 5 - 250 Valor
Reward 1 - 300 Valor
Reward 2 - 350 Valor
Reward 3 - 400 Valor
So on and so forth. Each time you get a reward, it sets it back to 0. So to get reward 1 the 2nd time, using the 50 example, would require 6 full runs again after getting the orb.
So to get that 60 writs once, which I believe is the 4th reward, you need 10 full arena wins. Then to get it a 2nd time would require another 35 full arena wins, and it keeps increasing like that.
Keep in mind, this is based on a made up example number of 50 per reward and is in no way a reflection of what will happen as I don’t know numbers of the rewards tiers