Actually, level 160.
Your three delve picks for the daily scrolls are based on your highest level ever completed in that particular delve, which includes inside the event and is (supposed to be) capped at 500. Your middle scroll pick (1.0 modifier start) will be whatever level it says your delve level is, seen at the bottom right of this UI element.
(If a level is not specified here, you haven’t completed a delve here yet, and you may to “advance” to level 20 with a 2.0 modifier start)
The level displayed here will be the same as on the faction summary screen under “Best Delve Defeated”, unless they allow it to break during the event again (like it did for @TheIdleOne for the last one, allowing it to go over 500 temporarily).
The events delve level is specified at the top middle of the screen:
If you don’t beat the boss event delve level remains static. If you do beat the boss, you advance to the next one in the event. However, this is similar to if you always picked the rightmost scroll option outside - you are advancing or attempting to the level specified at the top.
So, if you reached a displayed level 180 in the event, with the outside level previously at 100, you’d have just completed level 170 and your outside delve level (best delve level defeated) would be 170, with your scroll picks being 160, 170, and 180.
However, if you beat the boss exactly 15 times inside the event, your highest level beaten would only be 160, and your outside delve level would be 160. Because you would have won at 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, and 160, so your highest level beaten here is 160.
Side note, if you started a delve at a low level with a scroll and then progressed in the event, that one delve will stay at the level you started it at. For example, I progressed my All-Seeing Eye to delve level 60, but I still had an unfinished level 30 2.0 modifier start. When I ended that run, my choices were 50, 60, or 70 because my delve level was 60 having already beaten the boss at 60 inside the event.
To stay at level 100 HoG delve level with an expected total sigils of 15 (including Valravens, which is within low range for the starting sigils of 9 with t4 weapon purchase), for maximum loot, clear all other rooms for 6 sigils and then Run Away, then clear all rooms and beat the boss on your last 9 runs. This is also way more than enough event points to reach Reward 8, which can be done with a t4 and running away the boss every time (22000 points total needed, est 1.9k-2.1k pts for all the rooms before the boss +~700-750 per boss kill, I ended at 35k for ASE with 5 boss kills and t4 purcahse). If you end up with 2 sigls left after you have done this (17 is also within expected range for 9 starting sigils), you don’t want to beat the level 110 boss inside the event if you want to keep your level 100 delve outside, but you can still Run Away after clearing other rooms without advancing further (it just would have been easier/faster to do it at level 20, also less chance of having a valraven fly away before you can kill it).