What you’re doing is “clearing rollover via matched betting / hedging between two books”. It’s not really scalping in the traditional “beating slow line moves for risk-free profit” sense; it’s more like bonus / rollover arbitrage.
Here's my advice:
You have Book A with $7,800 remaining rollover. $1,150 balance you'd like to lose here.
Book B: No rollover, you'd like to win here.
The ideal would be you see a dog at Book A let's say at +300 and the favourite at Book B at around -300. The closer the implied probabilities sum to 100%, the smaller your loss.
At Book A (rollover) take the side with the bigger price. At Book B, bet the opposite outcome.
Use an arb/matched bet calculator if you want exact stakes.