I need some simple roll table help please

So for the campaign i am running i want RNG to be decent, so if i were to create a roll table (i have already, but) what values would i give my items to make rare items and common items balanced enough? i was thinking of doing a 100-1 thing where really common items will have really high numbers and rare items will have pretty low items, but how do i tweak that to be exact? i realize its probably just gonna take trial and error, and worst case scenario if it doesnt spit out rare items i know i can just hand them out, but if you have and experience/tips about it, that would be great, thanks! :)

It will really come down to personal preference. If the only criteria you have is common vs. rare, I'd suggest thinking about how many common items you would want the players to get compared to how many rare items, then set the weight based on that ratio. So, assuming you want 1 item in 10 to be rare, you would make sure the sum of all your common item's weights was 9 times higher than your rare items. I'm not sure what else there is to say on the matter.

I tend to do my tables of this sort by percentage (ie, have all your weights add up to 100 in total) that way you can quickly and easily decide on your percentage chance of something common or rare happening. In the example Aaron gave of one in ten being a rare item you would give the rare item a weight of 10 while the rest of the items in the table have values that add up to 90. For me, i tend to have 50% of the table taken up with various amount of coinage/gems/common loot etc, a further 30% being more exciting and expensive items but potentially bulky therefore creating a profit/weight issue for my players and the final 20% being the rarer or specialised items (with a subsection of that final 20% being weighted as 1 for the truly 1 in a 100 items lol). Hope that helps a bit... but table are limitless so like Aaron says, either by weight or percentage its all about ratios :)

i really like the idea of making the weight = 100, i think ill use that, thank you very much!