not only is this fine maths, it's also an insight as to how hits and damage are calculated.
It also confirms what I've been thinking for some time. Every time you hit a monster, your attack level (plus prayer boost and slayer helmet effect or salve ammy) added to your gear attack bonus (slash, stab or crush) gives a number (let's say 250 maxed out with godsword). From this number, a random number is rolled. Let's say you rolled 185. On that hit, your enemy will roll a random number from 0 to ... whatever his max bonus is. If your enemy rolls 185 and higher, you splash a 0. If it rolls under 185, you will get a hit. The hit is determine by the str bonus and another random number is rolled translating to damage. Note that you could succeed with your attack roll and hit a 0 with your str roll. That's still hypothetical, but I'm pretty sure that's the most accurate explanation.
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