G'day Benny, I'm not the ultimate ekspert but I'm pretty long in the tooth.

Your figures are very realistic, disected = 210 bets for a return of 220 units,
or about 5% POT. or top 3, 315 bets for a return of 342 = about 8% POT

At that level I would use the retirement staking plan (google it) counting either your top 2 as 1 bet or your top 3 as one bet. i.e. say your next bet is $30 so stake $10 on each (backing 3 per race that is) now you have to make the decision whether to back and recalulate race to race OR day to day (my preference) in other words say you have 5 races rated today and your next investment on the retirement plan is 45 units so you have 3 units win on each selection in each race, (always getting the best price possible of course)

At the end of the day recalculate what the next days bet is and repeat process. You should lift your POT to around 20% +