To win a set amount you need to control a couple of things.
Let's assume your target is 1 unit (dollars, pounds or rupees) and you want to trade out at 1 tick below your back price.
The back price is $3.50. Therefore 1 tick is 0.05.
The first is the initial amount you wager.
target/(1-commission)/b_tick*back_price
1/.95/.05*3.5=73.68
We now know your lay price, 1 tick below the backed price, 3.45
bm_backp/r_ticks(bm_backp,-1)*bm_backa
3.5/3.45*73.68=74.75
You now submit a lay offer of 74.75 @ 3.45
If/When matched you have a arbitrage of +1.07 - commission is about +1.01.
In your example you need to wager 47.37 @ 1.8
Then lay 50.15 @ 1.7, you trade out with +2.78 less .14 = +2.64
You see prices collapse regularly by 10 ticks or more, the tough part is predicting a trend continues or changes direction you need before it happens.
Good luck.