They did well to keep the filter housing horizontal when the top lug was no where near.
I have always held onto the housing with one hand as I tighten it, obviously when located correctly. It can be rocked as it's tightened until you can only just feel it touching the crankcase as the o-ring seal gets compressed, then tighten no more. Perfect seal, with not too much stress to allow them to be undone without recourse to rounding the 12mm head off 
I always use the Honda specified torque setting on this bolt Nigel but is your method more suitable to avoid the dreaded 12mm original bolt head rounding off? I would normally fit the aftermarket 17mm bolt but on ratty I'm going standard = more risk to rounding off at some point in the future.
To expand Dave, obviously Honda give a torque and rightly so. The range is not that tight (for 750 it's 19.5~23.9 ft/lb) and maybe not so repeatable if there's any variation in equipment too.
It is alternative but completely accurate to do it by feel as you'd never want to load the filter housing further than when the seal is effectively fully home. By "rocking" I meant rotation within the tab and it's restriction by rocking it clock and anti clockwise while tightening.
Takes variable out of equation in reality by bringing the o-ring to compression and effective seal while leaving the housing the least stressed from the bolt.
Generally, methods like this are used in more critical cases to take out variation in tools and calibration, plus oil on threads etc.to reach the same point of joint security.
The tab and restriction on cases stops the filter housing backing out under vibration so they've designed it such that it can't start to rotate and unwind the bolt with it.
Obviously a number of functions being performed by the bolt, seal, filter and housing, making a balance between the design elements a little more complex. They put a 12mm on it to try and prevent any significant risk of tightening too much in workshops as it's a 20mm bolt with consequential risk to the cases and housing. I've owned a six sided snap, on socket for them since tge 1970s
also a 17mm for tappet caps.
A long time ago I tested method against torque to see the difference, which came to the lightest torque Honda gave, but current o~ring material may shift that a little. Probably worthwhile doing your own comparison to verify. I've never had a leak but often had to get other's filter bolts off after they've stuck, a couple of hairline cracked housing too.
