It's pretty much what SniperDuck said; there are several base values, one for each type of "item roll chance".
For example:
Base chance to get a painted item = 8 (1 in 8 items)
Your "better chance to get a painted item" percentage = 25%
New chance = base / ( 1 + (percentage / 100)) = 8 / (1 + 0.25) = 6.4 (1 in 6.4 items, or 5 in 32 items)
Other examples:
Base chance to get a magic item = 5 / 2 (1 in 5 items to get a suffix and 1 in 5 items to get a prefix)
New chance (with 30% better chance) = 5 / 2 / (1 + 0.3) = 1.92
New chance (with 50% better chance) = 5 / 2 / (1 + 0.5) = 1.667
New chance (with 75% better chance) = 5 / 2 / (1 + 0.75) = 1.43
New chance (with 150% better chance) = 5 / 2 / (1 + 1.5) = 1 (item will always have at least a suffix or a prefix)