Click at the left then the right edge of each baseline. A line will appear, and the co-ordinates will be added to the table below. You can nudge the left endpoint around with Shift + arrow keys, or the right with Control + arrow keys. Create a new estimated line with Alt+down arrow.
| Left end X | Left end Y | Right end X | Right end Y |
|---|
| x1 | y1 | x2 | y2 |