(0.02 / 218) * (395 / 2) = 0.018 BTC
Question: I am earning 0.018 BTC in average? or per year?
It depends on where those 395 BTC comes from. I guess it is total profit (starting from 2016) so its "per 4 years". But I would not take such long time frame bacause we dont know how big bankroll was than. It could be 50 BTC not 200.
You can do it like that:
Profit in 24.06.2019 - 335 BTC
Profit now - 395 BTC
1 year profit - 60 BTC
(0.02/218) * (60/2) = 0.00275
0.00275 / 0.02 = 13.75 % annual ROI
But this estimation does not include bankroll change. Its 218 BTC, it can go to 400 in 1 year or drop to 50.
Calculating the same (annual ROI) based on old investors resoults:
We have 11.75% in 6 months - 24% annualy -so it looks like during last 6 months total invesments at least double or triple.