According to several sites the First Haste plateau is roughly 25% (this number has nothing to due with my question). If I need 25%, do I only need 20% due to the buff from Shadowform to each "x" plateau.
yes, although the numbers are different. the breakpoint mentioned on most sites is after you've been fully buffed.
for races that aren't goblin you want 8085 rating, which assumes you are in shadowform (5% haste buff).
Keep in mind that whilst not particularly relevant as a spriest (since you provide spell haste yourself), having 20% haste then gaining a 5% haste buff will actually result in you having 26% haste. Haste is multiplicative and not additive
You need an exact number, not a %. With how high number conversions have become rounding happens more often. For example: if you have 5099 hit, it will show up as 15%. You will still have that .000001~% chance to miss tho.