The amount of supply that goes to voting rewards is determined with the goal of having 90% of the total supply of ICP utility tokens locked in neurons. The process begins by deciding that 90% of the total supply of ICP should be locked in neurons. Then the required returns, in the form of newly minted ICP, are estimated to incentivize 90% participation. This allows the calculation of the total rewards that must be paid if successful. This quantity of newly minted ICP is then distributed regardless of the current level of participation, meaning that participants will receive larger rewards until participation reaches 90 percent.
The required returns are estimated as a percentage of the current supply, and this is allowed to fall over time to account for the risk of locking balances falling as the network becomes more stable. The distribution starts by giving out 10 percent of the supply (on an annualized basis), with this falling until 5 percent of the supply is distributed in eight years.
The formula for the annualized rewards as a percentage of total supply for the first 8 years is:
R(t) = 5% + 5% [(G + 8y – t)/8y]².
It's important to note that the total supply of ICP is a dynamic system with deflation and inflation, making it impossible to predict what voting rewards will be on any given day or year in the future.
For more details please read, Understanding the Internet Computer’s Network Nervous System, Neurons, and ICP Utility Tokens.