Thanks but I saw from schewser or either cfa curriculum that u need to reverse it. IS sell = real portfolio - paper formula. But it nvr srate whether need to reverse the formula . However for a sell trade implicit cost is BP - EP

Just use your logic, lets say you want to sell a script at $20, but delayed decision, it has gone down to $19.90, so formula and calculation methods remains same.

Lawence, the formulas would need to be reversed in the sense that since you are selling now, a gain would mean getting a higher price instead of a lower price which would be the case if you are purchasing. Do be careful, IS would still mean the differance between the gain on a paper portfolio and the real portfolio.