Yes, you can't just take your percentage of private shares and multiply it by the private valuation to estimate your IPO payout. But there are two reasons. The first which you identified: dilution. This happens at _each_ fundraise, because the company has to issue new shares, so your % will go down at each funding round.
The second reason is that the valuation of the company changes (typically, it should go up at each round, but in our current climate, down flat-rounds are very likely for many companies).
So your % ownership should go down, BUT if the valuation goes up by the right amount, the value of your ownership should go up too (ie your price-per-share goes up).
The second reason is that the valuation of the company changes (typically, it should go up at each round, but in our current climate, down flat-rounds are very likely for many companies).
So your % ownership should go down, BUT if the valuation goes up by the right amount, the value of your ownership should go up too (ie your price-per-share goes up).