even if you pay attention to the enemies hp/mr, you can mostly just guess where exactly the kill range of the ult is.
It's not difficult to make an educated guess. 30 MR means ~20-25% reduction (3/4 to 4/5 damage), 100 MR means half damage, 200 MR means 1/3 damage. Assuming max rank ulti, it's 375+40% of missing HP (400 damage per 1000 HP they're missing)
All you need to do is keep a few key milestones in mind; if they're missing 500 HP, +200 damage, 100 MR means it deals half as much...keep those minor details in mind, take a glance, and you should be able to determine when his ultimate will be a killing blow.
as far as the scaling goes, the base damage helps with executing squishies and allows to use the ult early against them if needed, and pick the kill up with ignite if they flash away.
I didn't say I had a problem with the base damage. It's the way that the scaling works. It's just not intuitive design; clarity is terrible and most people aren't capable of doing that kind of math on the fly.
there is still no reason, that you have to guess how much damage an ability actually deals. its different with flat values, where the calculation is much easier. every point of resistance effectively increases the enemies health against the damage type by 1%. so if you remember his mr or armor, its easy to quickly calculate how hard an ability hits.
but the addition of the % health scaling makes it unusual difficult to determine how effective the ability actually is.
Actually, each kind of %HP is not difficult at all to figure out. Missing HP is the most complicated, but only because you have to actually do some preliminary subtraction to figure out how much HP they're missing. After that, just round to the nearest 1000, multiply the % by 10 and you'll have your bonus damage.
What is difficult is on-the-fly calculations of their resistances and effective HP. But here's a trick; 100 Armor/MR = 1/2 damage. 200 = 1/3 damage. 300 = 1/4 damage. 400 = 1/5 damage.