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<blockquote data-quote="keterys" data-source="post: 4923736" data-attributes="member: 43019">If the math demonstrably works, why do you need the explanation? <img src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f642.png" class="smilie smilie--emoji" loading="lazy" width="64" height="64" alt=":)" title="Smile&nbsp; &nbsp; :)"&nbsp; data-smilie="1"data-shortname=":)" /><a href="http://www.enworld.org/forum/4e-fan-creations-house-rules/246700-players-roll-defense-against-npc-monster-attacks.html" target="_blank">http://www.enworld.org/forum/4e-fan-creations-house-rules/246700-players-roll-defense-against-npc-monster-attacks.html</a> was linked off this thread and shows it a little bit more, but basically...The average of 1d20 is 10.5 - you need to pay attention to where that .5 lands. Core rules only give defense +10, thereby giving the attacker a .5 bonus equivalent.When you say you have +10 and need to roll AC 20, you need to roll a 10. That's because you win ties you don't need to 'beat' the AC of 20 by getting a 21. When you reverse the equation, you need to account for the fact that the _attacker_ 'wins on ties'So, you had +10 vs. AC 20 is 55% chance to hit (need a 10)When you say you have AC +10 and you need beat an attack roll that would have be the equivalent chance, you need to bump up the DC for both the roundings involved (.5 on die, .5 on formula) and ties (+1)Attack = 20 (+10) would be a 45% chance to be hit, clearly off by 10% (or 2)Attack = 21 (+11) would be a 50% chance to be hit, clearly off by 5% (or 1)Attack = 22 (+12) is just right</blockquote>

[QUOTE="keterys, post: 4923736, member: 43019"] If the math demonstrably works, why do you need the explanation? :) [url]http://www.enworld.org/forum/4e-fan-creations-house-rules/246700-players-roll-defense-against-npc-monster-attacks.html[/url] was linked off this thread and shows it a little bit more, but basically... The average of 1d20 is 10.5 - you need to pay attention to where that .5 lands. Core rules only give defense +10, thereby giving the attacker a .5 bonus equivalent. When you say you have +10 and need to roll AC 20, you need to roll a 10. That's because you win ties you don't need to 'beat' the AC of 20 by getting a 21. When you reverse the equation, you need to account for the fact that the _attacker_ 'wins on ties' So, you had +10 vs. AC 20 is 55% chance to hit (need a 10) When you say you have AC +10 and you need beat an attack roll that would have be the equivalent chance, you need to bump up the DC for both the roundings involved (.5 on die, .5 on formula) and ties (+1) Attack = 20 (+10) would be a 45% chance to be hit, clearly off by 10% (or 2) Attack = 21 (+11) would be a 50% chance to be hit, clearly off by 5% (or 1) Attack = 22 (+12) is just right [/QUOTE]

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