I don't know what you're starting with - so I'll just make something up and build something from the ground up.

1) [CATEGORY SEARCH], I pick "Init Normal (AWM2)" for the Performance which is a non-velocity-responsive piano sound with no effects.
2) [PERFORMANCE] (HOME), then touch PART1 area and [EDIT] to start editing PART1 parameters
3) Touchscreen (TS): Menu "Effect" -> "Routing" -- from here I see InsA and InsB are set to "Thru" (no effects)
4) TS: Touch InsA "Type" area and choose "Comp Distortion" as the effect type.
5) I check to see that to the upper-left of InsA in the routing screen - the button is shown as "ON" (in green). By default it is for Init Normal (AWM2).
6) I play some notes which sound very distorted. I don't want this - I want the sound to start with no distortion and use the assign knob to add to a non-distorted value.
7) TS: Menu "Effect" -> "Ins A" to edit the Comp Distortion parameters
8) From this menu - the upper-left parameter "Overdrive" defaults to "52". When I turn the dial and play notes - I notice that from 0-30 is the area, to me, which seems like there is no (at 0) to "full" (at 30) distortion. Then after 30, the sound just seems to get louder. So I use my ears to tell me what my target range is. I'm going to aim for the curve to focus on 0-30. ** I set this parameter (Overdrive) to 0 so there is no overdrive by default and allows me to use a uni-polar (adding only) type curve.
9) With "Overdrive" selected, the [CONTROL ASSIGN] lights up telling me this parameter can be assigned as a destination. I press the [CONTROL ASSIGN] button, then spin Knob 2. - this brings me to the "Mod/Control" -> "Control Assign" screen with a default curve already applied to Overdrive.
10) I play some keys and hear distortion again. I notice Knob 2 is set to "the middle" (around 64) and not minimum. So I spin the knob to minimum - now I hear no overdrive. Good. Spinning from full counter clockwise (0) to full clockwise (127) - I hear a good amount of range (max is fine, min is fine) -- I just want the curve to be slower at first near 0 and speed up near 127 -- an exponential type curve vs. linear ramp. So I change Param 1 from "5" - which is default as a ramp - to "8"

Note that because volume does tend to increase for higher overdrive levels -- presets that use overdrive tend to adjust the volume of the PART downward at the same time as overdrive is increased in order to keep the level flat when increasing overdrive. This would be more of a requirement if the ratio for this curve was changed to 64 as the level would get twice as much - going further outside of the 0-30 range where overdrive gets very loud.

If you search [CATEGORY SEARCH] for presets with "Overdrive" in the name - you can discover which insertion effect is distortion then scan through the assignable knob source->destination assignments until you find "Ins? Overdrive" or "Ins? Distortion" (where ?=A or B) and see how the preset sets up distortion control.

EDIT: The knob I used for this is PART1 Assignable Knob 2. If I wanted the Common Assignable Knob 2 to control distortion, I'd touch the Performance Name from the home screen and [EDIT], then choose "Control" -> "Control Assign" menu, then setup AsgnKnob 2 as the source and Part 1 Assign 2 as the destination.

But If I assign AsgnKnob2 to an insert effect such as InsA OverDrv, the entire sound just just takes on the insert effect full hog and can't see how to control the level.
How do I do it so I can control the amount of effect applied to an element using knob2?

The nature of the routing has the entire signal traversing the Effect. Insertion Effects are "inserted" inline with the source.
By assigning control to AsgnKnob2 to OverDrv will allow you to determine the amount of Overdrive (which is just one controllable aspect of any Distortion Effect), I get from your question you are looking to control "the level", basically "how much" Distortion. Overdrive will change the character of the Distortion. DRY/WET will balance the clean with the dirt...

This can be controlled by creating another Source/ Destination setup to control the "Dry/Wet" balance.
For example, setup AsgnKnob3 to do DRY/WET balance

Enter the INSERTION EFFECT A
Highlight the "DRY/WET" parameter
The [CONTROL ASSIGN] button will glow meaning you can press it to make a Controller Assignment
Press [CONTROL ASSIGN]

Move the AsgnKnob you wish to assign this control.
Now you can control the character and the overall level of the effect.

You'll be taken to a screen where you can edit the application of the DRY/WET balance.
If you set the POLARITY to "Bi" and the (range) RATIO to +63 you'll have the full range of Dry thru to Wet.