Try this.
WIN BETTING ONLY
Approach No.1
Have a betting bank of $150 for every $1.00 win bet.Level stakes

Only bet on the critters showing $5.00+ in the paper.
Only bet on the Mules with TAB Nos. 1-10
(1-10 have a strike rate of 90%)

PLACE BETTING ONLY
Approach No.2
Try this staking plan from Eqestrean Investor
Bet sequence 1-2-6-18=$28 then stop & start again if you dont have a strike on that sequence.
You could have 6 banks of $28=$168

You said you have a place strike rate of 50%+
Test it on past results.

Leave out races with 7 & less runners.
Selections TAB No.1-10 only
No bet on selections paying less than $3.30 in the paper.
It should show a profit.

Then let us all know your findings.

__________________
Cheers.