To work out the liabilities , covert all prices to fractional odds, this is what bookmakers do.
Then divide ones exposure amount by the fractional odds.
e.g.
Say Lay $100 @ $4.00 (3/1) = $300 Liability (Pink side)
Say you wish to close out of this trade & win price is $3.80 (2.8/1) (Blue side)

We divide the $300 liability(Pink) by 2.8/1(Blue) = O/L 107.14 Blue
We bet $107.14 O/L on the Blue side

If the horse Wins, Ret $299.99w(Blue) -300L(Pink) = -$0.01 loss.
Approx break even % on the race if horse wins.

If horse Falls over, Ret $100 from lay bet (Pink) - 107.14 O/L (Lost-Blue)
= -$7.14
Approx -6.70% loss on race, if horse falls over

The exposure of Lay risk has been reduced from -$300 Liability
(your pay out amnt), if horse wins instead of falling over, down to just -1c if the horse should win.

It's enough to do ones nut in it is , but once one plays around with it a couple of thousand times , it starts to take shape in our brains.

Cheers.

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Cheers.