Now that we have mastered the arcane science of the world's best heuristic method to estimate slope dope, how would we live with this knowledge in real life? Even the best method is pointless without an action plan.

Let's start with border conditions, where heuristics still make sense. The logic is simple:

Here it comes.

EBRR?
or
Increasingly Improved Rifleman's Rule = IIRR? 2I2R = two-eye-two-arr?

Naming contest is still ongoing, suggestions are welcome.

When analyzing errors of Improved Rifleman's Rule (which appeared the most promising), I noticed that they are more or less (well enough for a heuristic method) linear in distance, at least for angles beyond ±20º.

GP11 Improved Rifleman's Rule errors, 0.1 mrad clicks

The next step towards perfection is attributed to señor Tiro Fijo of Paraguay. It's called "Improved Rifleman's Rule", the only difference being that correction is applied not to distance, but to correction. To wit,

1. Measure the inclination α;
2. Measure the slant distance (along LOS) D;
3. Look up bullet path (come-up) in ballistic table as if shooting at distance D
horizontally;
4. Multiply this correction (and not distance D) by cos(α).

And we get the following:

GP11 vertical miss distance, cm

I started with NDS method first because their other methodologies seem to be simple,
straightforward, and highly applicable. I consider their sniper/DMR training and Perotti's
book on sniping top of the line.

In that book, "From 1 to 1,000", inclined fire is covered by a couple of paragraphs that say approximately the following:

It's easy to google gory details on gyroscopic precession and Coriolis force and analyze every minute factor, but when (and if) is it worth doing?

Case study: standard army issue Fass 90 assault rifle, 300 m battle zero. How does it behave when shooting at an angle?

The Swiss army manual responds with exactly one concise sentence: "when shooting at an angle beyond ±30º, aim low". Not much for a mountainous country.