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.

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.