Turnover
The quarter billion will be the amount he puts through the market in bets, i.e. turnover, or throughput if you like. For example he'll place a £500 back bet, wait for the odds to move (hopefully in his favour) and then place a £500 lay bet. There's £1K towards the quarter billion, but he'll do that many times trading a single race, in about 50 races a day. Soon adds up. Good luck to him.