Notification at multiples minus a number


I have a spreadsheet where each row counts hours, then a column keeps a running total of all hours.

I would like to be notified at certain intervals as every 150 hours I need to do a task however I need some prep time so I would like to be notified 20 hours before 150.

So when my column value reaches 150, 300, 450, 600 I would like to be notified at 130, 280, 430, & 580.

While 130 is 150-20, I cant track factors of 130 since then I'll get notified 260, 390, & 520.

Any suggestions? thank you!


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