To both echo and add to what ctr wrote:

General consensus is plugs out, throttle wide open and + feed to the coil unplugged. I would add that, with an SU carb, you would likely want the dashpot off/open to get full, unrestricted airflow into the intake ports.

However, the most important thing is to recognize that a compression tester is a bit like a household bathroom scale. You will very likely get a different reading when you step on different scales, at different homes, wearing different attire, and at different times of the day. If you pick a repeatable methodology (like the previous advice, plus like engine cold or engine hot) then you will be able to have a picture of your engine's health as you go. Best used to compare your own engine to your own engine rather than one person's engine to yours.

Lots of variables such as is engine hot or cold? is there oil stuck to the cylinder walls from a drive an hour ago, or is it dry from sitting for a few weeks? How fresh is your battery?

Just be consistent and you'll get some meaningful relative numbers.

Note that, just like jumping on ANY bathroom scale while wearing ANYTHING will still be able to tell you , "Holy cow, I've got to lose some weight!" so too can a compression test tell you that there's something not-quite-right with your motor. If one cylinder is 10% or more different from the other cylinders, it can tell you that there is something different about it like a valve isn't sealing in that cyl as much as in another cyl. Or if you've got a very large discrepancy you can discover you have a blown head gasket.

So if you just want to rule out big time badness, there's nothing wrong with unplugging the + coil (or just putting the middle coil lead from the cap onto a head stud to ground it out) leaving plugs in, throttle closed while cranking. You'll just likely suck some atomized fuel into the ports if you crank for a while but it'll still tell you if you've got a blown head gasket or burnt valve.

hope this helps.