How to store data with backingfield in EF Core?

Question

I have the following class

public class InvestorsLottery : Lottery 
{
    [Column(TypeName = "decimal(14, 2)")]
    public decimal Investment { get; set; }
    [Required]
    public int Percentage { get; set; }
    [Required]
    [Column(TypeName = "decimal(30, 18)")]
    public decimal InvestmentEth 
    { 
        get { return _investmentEth; } 
        set { 
             _investmentEth = Investment / EthPrice; 
             value = _investmentEth
        } 
    }
    private decimal _investmentEth;
}

with the following fluent api

protected override void OnModelCreating(ModelBuilder builder)
{
    base.OnModelCreating(builder);

    builder.Entity<InvestorsLottery>()
           .Property(p => p.InvestmentEth)
           .HasField("_investmentEth")
           .UsePropertyAccessMode(PropertyAccessMode.Field);
}

but the value of InvestmentEth stays zero - how can I change this?