It really depends on season and latitude. In the U.K. I have never managed anything as dark as the picture shown by ROL. Even with a red and a polariser and the sun at its brightest and highest I have barely managed this dark a sky.

I just about got there with Ilford SFX which is a near IR film.

Unless you live in similar latitudes to ROL then I suspect you will be disappointed with what a deep yellow filter will do for you by comparison.

The light intensity at latitude 53 with a hazy-ish sky even on bright days as is the case in the U.K. means that what is possible with a yellow is quite different to what is possible in a clear atmosphere at maybe 20-23 degrees of latitude less.

Try a yellow then an orange and then a red and finally a red and a polariser together and see what you think. It'll only cost you four frames.I didn't notice any problem with stacking using two filters together

pentaxuser