I try to understand how long time I would need when flashing the paper, or burning the up half image without compromise the blacks.

Based in this test, it looks to me that I would need expose the paper for at least 3 sec, or burn the sky for 3 sec, in order to make the sky a bit light grey.

But I have a strong feeling that I am look at it in a very wrong way:

This 3sec dodge is what I actually did to come up with the result I posted before.