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
4. Multiply this correction (and not distance D) by cos(α).
And we get the following:
GP11 vertical miss distance, cm