... Balancing the light

Here I used a stack of 2 ND Gradual filters (Cokin P series) ND3+ND1
1 second exposure.
I did not use a fill flash for the ground right after me. (but it looks like).
The picture was shot longtime after sunset with poor light

Without the gradual filters I was getting an almost white sky a plain black silhouette and a dark ground before me.
With the filters I got that subtle orange (longtime after sunset), a lot more detail in the city and the ground before me has nearly the same light as the sky.

What did I do.?
I used a tripod and a wired remote shutter.
With the camera in (A) with an aperture of F13 pointed the focus only to the sky and I got a reading of 1/30"
With the camera pointed only to the ground (keeping (A) mode and the same aperture of F13) I got a reading of 1 second.
The difference between 1/30 and 1 second is 5 stops.
Deducted one stop and got a final result of 4 stops. So as I only have 1,2, and 3 stops filters stacked the 1 and the 3.
Turned the centre of the focus to the middle of the silhouette, changed to manual (M) kept the same f13 aperture and adjusted the filters keeping the transition from plain glass to the tinted area att the level of the bottom of the foliage

Shot with the reading I got for the ground (this is key) - 1 second

How to calculate the stops?
by just keeping doubling the faster speed till I got the longer speed. and counting the number of operations.

1/30 *2 = 1/15 (1 stop)
1/15 * 2 = 1/8 (2 stops)
1/8 * 2 = 1/4 (3 stops)
1/4 *2 = 1/2 (4 stops)
1/2 * 2 = 1 second (5 stops)

____
My experience is telling me now the most usual thing is to use the ND3 filter.. usually the difference between the ground and the sky is around 16 times more light in the sky.

Anyway I usually shoot with the ND3.. and check the result.. I add the ND1 or change to the ND2 accordingly to what I get. Or I keep the ND3 and go up and down with the speed.
When trying a shot like this is silly to shoot with only one settings. ( I still learning)