The wiki there says her E damage increases for every 1% with a maximum of 100% the minimum damage for her E at max rank is 120% of her AD
does this mean that it increases up to 220% and does 220% of her AD as true damage?
Yes I believe so
Furious Bite RANGE: 250 COOLDOWN: 12
Furious Bite Active: Rek'Sai bites her target, dealing physical damage that is increased by 1% for every 1% of her current Fury. If Rek'Sai has full Fury, Furious Bite deals true damage instead. Minimum Physical Damage: 80 / 90 / 100 / 110 / 120% AD
Tunnel COOLDOWN: 20 / 19.5 / 19 / 18.5 / 18 On-Target Cooldown: 10 / 9 / 8 / 7 / 6
Active: Rek'Sai dashes in the direction of the cursor, leaving behind two connected Tunnel.png tunnel entrances. She can interact with either entrance to traverse the tunnel again, but each tunnel shuts down for a few seconds after being used, momentarily preventing further traversal. Rek'Sai's tunnels last for up to 10 minutes and can have up to 8 pairs of Tunnel.png tunnel entrances active on the map at the same time. Enemy champions can destroy a tunnel by standing on either entrance for 1.5 seconds.Ability Details Furious Bite is a single-targeted ability. Tunnel is a targeted dash that leaves behind a Tunnel unit. ◾Furious Bite applies spell effects as a single target ability. ◾Hextech Revolver item.png Spell vamp is fully applied. ◾Rylais Crystal Scepter will apply a 40% slow for 1.5 seconds. ◾Banshees Veil Spell shields will block Furious Bite.
Additional Information: ◾ Enemy champions that destroy a tunnel are granted 5 gold each. ◾ Although Rek'Sai can use a tunnel both when she is burrowed and unburrowed, she is considered to be burrowed while inside one - she will gain her burrowed skill set (although it is unusable), have reduced vision and have access to Tremor Sense. She will resume her previous stance when the tunnel ends. ◾ The healing from Fury will not take place while using a tunnel unless Rek'Sai was burrowed prior to entering it. ◾ Tunnels that are destroyed while Rek'Sai is in transit will not prevent her from completing the movement. ◾ Tunnel entrances are always visible on the minimap to allied champions. Enemy champions must sight an entrance at least once before it becomes visible and sighting one entrance will not automatically reveal the location of the other. ◾ Tunnel formation behaves similarly to blink abilities in that if the endpoint tunnel would be in impassable terrain, Rek'Sai will instead create the endpoint tunnel at the closest available spot of passable terrain. ◾ Tunnels behave like valid targetable units for the purposes of Teleport.png Teleport, Safeguard.png Safeguard, Shunpo.png Shunpo and Leap Strike.png Leap Strike. ◾ Rek'Sai can interact with tunnel entrances even if there is terrain between them, as long as the entrance is close to the wall and the wall is sufficiently thin.
My evaluation is that you are trying to find out about the unburrowed form. The damage starts at (80/90/100/110/120)% of her AD and increases by 1%-100% of that based on her rage. Meaning at 100% rage she does (120%+(120%x100%))=240% AD as true damage.
At 1% fury she does (120%+(1%x120%))=121.2%
