Just configure the logs deletion on the specific date

It just a notification alarm that you exceed or meet the limit of it you can just add up a little and for the logs you can set it automatically delete

Because you just set an alarm to them that if they exceed this limit then you as an administrator get an email notification or SMS etc it is called soft threshold whereas in hard method they not only notify you but will stop data from saving.

You can manually set a higher threshold for the logs make sure that you have a backup logs in email when setting auto deletion on the appliance

Just enable the automatic deletion and add a little of thread hold

It is only notification on the threshold. You can set it higher and the days of deletion of the logs

You can set manually the automatic deletion of the logs

Alarm threshold if the limit exceed and will automatically deleted on user preference

it can be deleted automatically after days setting

It is just only a alarm threshold

1. According to your first point, you are using the default setting where two options are selected only for Alarm not for auto deletion of logs, Disk Usage Alarm Options (i) 180 days and (ii) 85% Threshold it only gives you notification.  This is the reason your threshold is 87% which will not exceed to 90% because:
2.  According to your second point, you cannot set the threshold percentage because its automatically disable but you can set the days for auto deletion which will clear the disk after that days.