Hi
Yes it will DL at best available but there is always a minimum of 1 tick difference, so unless your conditions stop the DL, it will always be at least 1 tick loss.
A true break even scratch is same price and same amount as entry bet.
For Lay entry scratch is:
back, price: bm_layp, amount: bm_laya, plus conditions to suit your scratch
For Back entry scratch is:
lay, price: bm_backp, amount: bm_backa, plus conditions to suit your scratch