you cant without knowing the spreads. RNG and loot tables work this: RNG is just a random number generator, when you beat a boss or trigger a reward event RNG rolls a random number. This random number then makes a call on the "Drop table" for that boss or reward event. A Drop table works as follows: There is a table of rewards say gun 1, gun 2 , piece of armor, and some worthless crap. The drops will be listed on a numbered table, for this example 1-100 1-25= worthless crap. 26-27- gun 1 28-45=wothless crap 46=gun 2 47-75=worthless crap 76-85=piece of armor 86-100= worthless crap. The RNG rolls a random number, but the results are entirely dependant on how the drop tables are populated.

This^^ But this discussion is about Xûr. It's entirely possible that his loot table is weighted such as this, but without knowing, one assumes it's weighted equally. The OP's math is sound for an equally weighted loot table. Only Bungie can confirm this though, and I wouldn't count on it. Reading this thread has definitely lowered my faith in the future of humanity. Oh well, luckily the probability of my death is 100%.