I think until the IPO shares are sold out, all mined BTC should be paid out as dividends (after subtracting expenses).  IPO shares are currently being sold to raise capital for rigs, and paying the maximum dividend will help sell those shares more quickly.

After all the IPO shares are sold out, maybe 50% of mined BTC could go to purchasing new rigs (growth) and 50% could be paid out as dividends?