They could make this simple by adding the display for it after the cap

300/300 (+342)

Which means you are at 300 of the 300 cap and have 342 to roll over on the next reset. After the next reset you would be 300/300(+42).

This would make it nice for people who take a break for a week or have to work overtime etc. If we skip BCB for a week and don't run dungeons and have 300+ rollover myth we wouldn't miss are cap for the week.