I remember reading that the rule of thumb was to have an attack rating vs PC 2x the defense rating. Is this standard across the board for all kingdoms? For example if Kingdom 1 has a rating of 20k vs PC would they have essentially the same chance as any other Kingdom with a 20k rated group? I ask because I have been attacking PCs with less than the 2x rating but frequently the battles are ending too fast to actually develop veteran troops. I believe that promotion is really based on battle length and damage taken.. so if my troops have great defense should I try to use smaller groups? Perhaps closer to a 1.25 to 1 ration when attacking?
Group make up has a huge effect as does many other factor. I would love to see the battle has not lasted long enough thing go away and stick with just the 5% rule. I have had battle where I lost 60% of my force, leaders, and wizards but learn nothing from it as it did not last long enough.
Kingdoms like the elves or others with lots of archers must loose sometimes 30% to have the battle last long enough or they end it because there archery is so much stronger than other kingdoms.
For your question though. Kingdoms that have high archery values or wizards may need to attack at a lower ratio to get promotions but then will suffer higher losses because once they hit the infantry stages they get chewed up more. Kingdoms with mostly infantry type troops 2:1 or 2.5:1 is a good number to shoot for. I would really try to not go below 1.5:1 for any battle.
I have noticed many times that my leaders may or may not get promoted attacking villages but almost always promote troops when attacking towns. It seems to be a trade off. Advance your leaders and troops and suffer the attrition or play it safe and capture the PC with few losses. The RD does not need a 2v1 ratio, i would say you are close with your 1.25 with villages and maybe 1.5 for towns for that advancement you seek.