Count number of empty cells since last non-empty cell

Here's the setup I'm working with.

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It's a work out plan. Rows are dates, columns are exercises. Every time I complete the exercise "back" I insert a cross at the respective date.

I need a way to count the rows since the last time I did a particular exercise. In the example, if it were currently 9/16 the solution should yield 9 days for the exercise "back", 1 day for the exercise "front" and 8 days for the exercise "oh". I should be able to apply the solution on the whole column "back". There might be several check marks in one column, I am only interested in the number of empty rows since the last check mark.

Any help? Thanks.