The broad, short version is that there are 192 days in this year's NHL season. The cap is calculated on a daily basis and a player's daily cap hit is 1/192 of his AAV. Let's use a $1M AAV as the example. Every day a $1M player is on your NHL roster, he counts for $5,208.33 against the cap.
The $88M cap means that you can spend $88M of cap hits throughout the 192 day season. 1/192 of $88M is $458,333.33. On day 1, you are not allowed to to ice a roster with a combined daily cap hit more than $458,333.33 (excluding LTIR, which I am ignoring for this purpose). However, if you spend less than that amount on day 1, then you 'bank' cap space to use later. Let's say that you have exactly $408,333.33 in daily cap hits on day 1. That means that you have an 'extra' $50k to spend across the remaining 191 days of the season. If you have exactly $408,333.33 in daily cap hits for the first 60 games of the season, then you have 'banked' $50k per day, for a total of $3M of cap space to use in the future.
Ultimately, tracking this daily is a ton of work.
Capwages has an (IMO imperfect) daily tracker. I don't think they do a great job fully tracking/explaining how LTIR actually impacts the daily calculation and a team fully relying on it wouldn't maximize how well they could maneuver the cap. But for our purposes, it is plenty accurate.
However you slice it though, the Blues have the ability to call anyone up. We have a cushion of a few hundred grand of daily cap hits before we would even start to utilize the relief from putting Krug on LTIR and those AHL call ups only count for about $5k a day.