Using sheetnode cell value as field

I want to add a field (field_invite_count) to a content type (machine name: guest_list) that already contains a sheetnode (machine name: field_guest_list).

The invite count field would display a value (A4 in this case) from the sheetnode. It seems I can use computed fields to do this, but I am not sure how to write the PHP to reference the cell.

I am not sure if it matters, but the cell in question (A4) contains a formula that will change depending on the content of the sheetnode. Will everything update automatically or upon save?

There is already a similar, but more more complicated looking issue here, but I don't know PHP and can't figure out how to adapt it for my needs.