Well, they say to check it at pin 5 of P900 (big connector on the HV cage), but if you'll notice, that pin has an orange wire that goes to a connector on the neckboard. It's much easier to measure on the neckboard where that wire is connected. It should measure approximately +180vdc. You can adjust the HV Adjustment pot inside the HV cage if you need to dial it in, but if it's way off (like +300), then it's most likely that adjusting the HV pot will not help - and you have some other repairs to do.
BTW - take the HV cage cover off when adjusting it, unless you have a long plastic screwdriver to stick down the hole....