I'm not sure that your numbers account for share dilution. Let's assume the employee stock pool is 10% before any funding is raised so the founders start with 90% and the employees 10%. Assuming your numbers above are correct, the founders are left with 72% after the first round of investment (500k at 2.5m = 20%) and the employees are left with 8% (think 72 + 8 + 20 = 100). The second round of investment takes 35% of the company, leaving the founders with 46.8%, employees with 5.2%, first investors with 13% and the new investors with 35% (46.8 + 5.2 + 13 + 35 = 100). The third round of investment takes 8% of the company leaving the founders with 43.056%, the employees with 4.784%, the first investors with 11.96%, the second investors with 32.2 and the new investors with 8% (43.056 + 4.784 + 11.96 + 32.2 = 100).
If my math is correct, the founders may have ended up with more like $430,560,000 between them ($215,280,000 each assuming 50/50).
I was just doing some rough calculations to show an approximate 'low-figure' of their take.
It is very possible and likely that they took away much more (because all of the variables could have changed).
For instance, that first $500K round could have been convertible debt which would have converted in the $20M round. If that's the case, then those investors ended up with 2.5% instead of 20%. That drastically changes the math and gives the founders more equity and a better outcome.
If my math is correct, the founders may have ended up with more like $430,560,000 between them ($215,280,000 each assuming 50/50).