Already available

Other crypto-tax software provides specific reports, and helps you understand how much you need to sell to realize the loss. To do the equivalent with Koinly, I had to create a bunch of fake manual transactions and look at the tax results, then delete them.

This is already shown on your dashboard. It shows your cost basis (colst column), the current value (market value column), and the percent up or down. When it is down is the time to harvest (sell/rebuy) as desired.