Re: Calibrate the time base, or horizontal amplifier?
The "official" way is to use a 067-0680-00 calibration plug-in (https://w140.com/tekwiki/wiki/067-0680-00, the 067-0587-xx is the same for the 7000 series) to adjust the mainframe first.
Without one, you could in principle feed a signal in through the amplifier, and adjust its amplitude (as measured on the scope's backplane) to the specified V/div - but you'd have to be really sure you know how to get this right.
The '680 produces a stair-step signal that really helps getting the geometry right, of course a suitably configured arbitrary generator could be substituted.
I would still recommend the '680, maybe you can borrow one from someone on the list.