Hi, maybe can help.
Can develop perdiction code for google spreadsheet (spreadsheet in cloud, you do not need install just input developed code) with using Betfair Api.
Not clear what do you mean about Harville Formula and not sure if it give you accurate result, most of hourse finishing have random nature.
One solution, in few words, is analyse previous hourse results together with bid value correlation for certain event.
However, this is complicated matter demand time for algorithm/strategy developmen and which don't give you 100% accurate result.
Regards, Viktor