Uptime Score calculation

Where exactly is the formula U = counter/ (epoch_size - 11) explained in more detail?

The spreadsheet that has been posted on the discord, uses the following formula:
epochSize - 11 - (blocksNotSigned - 12) / (epochSize - 11)

I was wondering why it wouldn’t be (blocksNotSigned - 11) which is how I interpret the the docs?

For a validator to be ‘up’ at a given block, it must have its signature included in at least one in the previous twelve blocks.