OK for all those that don't quite get bonus I'm going to simplify it :
You invest 1 BTC on first day with 20% bonus and let's for the purpose of this calculation say 1 BTC = 10000 WAVES
First day 20% bonus you would get 1.2BTC equivalent of WAVES = 12000 WAVES
Second day and till the end of 10% bonus you would get with 1.1BTC equivalent of WAVES = 11000 WAVES
No bonus with 1 BTC would be same so 1 BTC = 10000 WAVES
So those investing the same 1 BTC later when there is no bonus will get less waves than those that invested yesterday for 20% or 10% that starts today.
Nope...
There's 85M Waves for the ICO.
So the first 24 hours everyone gets the 20% bonus, right?
Let's say 4000 BTC got brought in, that's 21,500 Waves per BTC.
But there can't be a 20% bonus yet, because you would need 102M Waves to do that, but there's only 85M available.
Imagine someone putting 50,000 BTC in at the 10% phase? ...
Only in the end of the ICO you can calculate / know your real bonus.