Every step taken to reduce visible cost — resting instead of taking, slicing over hours, setting a tight limit — raises the chance of not trading at all. The unfilled remainder is invisible in a fill report and can easily be the largest cost of the order.
Any honest cost framework, implementation-shortfall included, charges the unfilled quantity at the price move you missed.
Example: you want 50,000 shares at 25.00 and fill 18,000 at an excellent average of 24.99. The stock closes at 26.40. The 32,000 unfilled shares represent $44,800 of foregone gain, against the $180 you saved by being patient on the part you did get.
Related: implementation-shortfall, fill-rate, arrival-price, execution-quality