The option for repoting checks how many levels dropped since drinking the potion, so if you are level 99, that's a +19 boost and the setting to keep it at +15 or higher would be from 4 to 4 (repot when stats dropped by) but it's important to mention that this will take ALL 3 stats into consideration for super combat potion
In your case, drinking every 15 minutes means that the potion must be able to boost more than 15 levels, so this won't work if the stats aren't high enough; if you are 99 and receive a +19 boost, you can set it to drink after stats dropped by 15 to 15 levels (less than 103, original boost was 118)