If I have 90% of a level, and the week resets will I gain a level? or does the ratio stay the same and the amount overall is reduced?
Every week the amount of azerite required to reach the next level is reduced by 30%. The amount of azerite power you currently have is also reduced by 30% so that you remain at the same percentage towards your next level on the Tuesday reset.
For example, let's say you end the week at 700/1000 (70%) azerite power. When Tuesday rolls around, you will find yourself at 490/700 (70%) azerite power.
As a result, it is good strategy to save as many azerite power turn-ins as possible until the reset to avoid the 30% reduction.