How to count the number of blank cells in one column based on the first blank row in another column

I have a spreadsheet set up with tv program titles in column B, the next 20 or so columns are tracking different information about that title. I need to count the number of blank cells in column R relating to the range in column B that contains titles (ie, up to the first blank row in column B.)

I can easily set up a formula to count the number of empty cells in a given range in column R, the problem is as I add more titles to the sheet I would have to keep updating the range in the formula [a simple =COUNTIF(R3:R1108, "")]. I've done a little googling of the problem but haven't quite found anything that fits the situation. I thought I would be able to get the following to work but I didn't fully understand what was going on with them and they weren't giving the expected results.

I've tried these formulas:

=ArrayFormula(sum(MIN("B3:B"&MIN(IF((R3:R)>"",ROW(B3:B)-1)))))

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=ArrayFormula(sum(INDIRECT("B3:B"&MIN(IF((R3:R)>"",ROW(B3:B)-1)))))

And

=if(SUM(B3:B)="","",SUM(R3:R))

All of the above formulas give "0" as the result. Based on the COUNTIF formula I have set up it should be 840, which is a number I would expect. Currently, there are 1106 rows containing data and 840 is a reasonable number to expect in this situation.