Question on making an incremental flying speed.

Recently i had an idea for a homebrew species. One of the traits for this species is a flying speed and i wanted this flying speed to start kind of slow and slowly get faster as you leveled up. The idea is that their flying speed is going to be equal to their walking speed at levels 1-7, at levels 8-14 the flying speed increases to one and a half times their walking speed, and at level 15 this flying speed becomes twice their walking speed. I tried doing this a couple of times now and the results were the following; the bonuses for future levels were still being applied at level 1 even though i used the level restriction for the trait, and for the second it just didn't work at all. Does anyone have any tips/ideas for how one can implement this?

TLDR. I'm trying to make a species with a flying speed that increases by character level. I've tried a couple times and failed. How would i do this on DNDBeyond?

In my subclass, Kinetic.Vanguard, I did that.as.a.skill, I tied my flight speed bonus to proficiency bonus so it scales naturally with player level. Hope that helps!