What you are doing is very nice and I think 3.5 years are much higher specially in the case where in past two years, bitcoin changed a lot and have very high value, so you should change this method atleast.
Thanks
The value has changed greatly, but the way that the price behaves in terms of percent changes has actually been pretty consistent. I ran into problems related to this when I first started working on neural networks to predict bitcoin prices, and I was able to solve them by looking at percent changes in price instead of actual prices.