I have not used graduated neutral density filters to help control contrast in my landscape photographs since switching to digital 15 years ago. Instead I prefer to shoot a series of different exposures to capture the full dynamic range of the scene and then either blend them using layers in Photoshop CC or, if possible, by using the merge to HDR function in Lightroom CC.
As I always shoot in manual exposure mode it's easy to determine how many exposures are necessary to capture the full range of tones in a scene simply by viewing the live histogram. However, there is a trick that makes this even easier if you use a Canon camera body.
1.Make sure exposure bracketing is set to -0+ in the menu. This means the sequence will begin with the darkest exposure for the highlights and finish with the lightest exposure for the shadows. *
2.Set your bracketing sequence to 3, 5 or 7 shots, depending upon the amount of contrast in the scene. **
3.Set bracketing to either 1/3rd, 2/3rd or 1 stop increments. ***
4.Turn on Live View and bring up the live histogram. ****
5.Set ISO and the aperture you need for sufficient depth of field.
6.Now the clever bit - if you half press the shutter button with your middle finger Live View (and the live histogram) will show you the resulting exposure for the darkest frame in your sequence with the current exposure settings. With your middle finger still half pressing the shutter button, use your index finger to adjust the shutter speed dial. Take it to the point where there is a decent gap on the right of the histogram. You have now set your darkest exposure in the sequence (to capture the highlights in the scene). *****
7.You can now fully press the shutter button. The whole sequence of images will be taken starting with the darkest exposure that you have just set for the highlights. The exposure time will increase in either 1/3rd, 2/3rd or 1 stop increments for as many frames as you have set.
8.Check the final exposure (for the shadows) to ensure sufficient detail (there should be a decent gap on the left of the histogram). If it looks ok then you know you have all the necessary exposures to produce a good exposure blend or HDR image. If the final exposure is too dark you will need to either increase the number of bracketed shots (if possible) or increase the gain between each exposure (to perhaps 1 1/3 stops or more).
* This setting will remain held in the menu so you only have to set it the first time.
** I normally leave my camera set to 7 shots and only use the exposures that I need from each sequence.
*** I tend to use 1 stop increments most of the time.
**** Most modern Canon DSLRs have a live histogram - repeatedly press the info button until it appears.
***** Works best with the top control dial set to select shutter speed and the rear control dial set to select to aperture (the default on Canon cameras).
I normally have my camera's drive mode set to self timer with 2-second delay. This means that with one full press of the shutter button the whole sequence of images are taken. Alternatively you could use the high speed motor drive in combination with a remote release.
One of the main benefits of this trick is that it speeds up the image making sequence, which is an important consideration if you have fast moving clouds in the sky. By utilising the motor drive your sequence of images is taken very quickly, minimising any movement in the clouds between shots and therefore making it much easier to achieve a good blend without ghosting. I use this technique for all my bracketed sequences as it speeds up my workflow in the field considerably.