Lets say you create this devotion aura with 11 levels to give bonus armor
Lvl 1: 0 bonus
Lvl 2: 10% bonus
Lvl 3: 20% bonus
...
Lvl 11: 100% bonus armor.
Now, lets assume your Avatar increases armor by 10% of hero str by level, and has 4 levels, and the hero has 98 str. It would increase armor by 10%, 20%, 30%, and 40% of Hero strength
98*10% = 9,8% armor increase
98*20% = 19,6% armor increase
98*30% = 29,4% armor increase
98*40% = 39,2% armor increase
When your unit casts the ability, you have to set the ArmorAura ability to it's corresponding %. I'll assume that the ability level is 4, so it increases amor by 40%.
Set ABILITY = Ability Being Cast (Avatar)
Set ABILITYLEVEL = Level of ABILITY (4)
Set UNIT = Triggering Unit (The Hero)
Set STR = Hero Strength of UNIT (98)
Set REAL = (ABILITYLEVEL / 10) * STR = (4/10) * 98 = 39,2.
It should increase armor by 39,2 but it will round to 40, making the DevotionAura level to 5 (because lvl 1 gives 0)
Now conver REAL to Integer and it will be 39 (I think). Now add 10. It will be 49. Now divide it by 10, it will be 4,9, wich is 4 or 5 (i'm not sure if REAL to Integer conversion rounds up or down). Now set level of DevotionAura to the Now proceesed REAL, which is 5, which gives 40% armor. You can then start a timer and set the ability back to level 1 when it expires.
It's not really polished, I'd prefer you to learn a bit about Wc3 coding logic and some math rather than doing this for you. It's always better to learn how to fish, than to fish to feed someone else every day.