Percent Max Health AP Damage Question

I enjoy playing champions that are against the Meta in blind with my friends. Some champions have rather unique scalings that don't apply when they are played by the meta. For example, both Nasus and Varus have abilities that do damage to enemies based on their %max health, but the amount of max health goes up the more AP you have.

What I'm wondering is how the damage is calculated. According to the tool tip on the ability (Varus), the percentage point changes every time you pass 25, 75, 125, etc, rounding to the closest multiple of 50. Therefore, there is no difference in damage on Blighted Quiver if you have 75 AP or 124 AP (Both round to 100, which is 2%)

This is significant because the ability stacks 3 times, so a 1% difference turns into a 3% difference, which on a tank can be around 100 damage.

What I'm wondering is: Is this the way the damage is calculated? With integer percentages? Or is the percentage calculated as a decimal value that isn't shown in the tool tip?

TL;DR Are %Max health abilities that increase with AP (1% per 50 AP) calculated as integers or do they go to decimal percentages?