I wouldn't have thought being at a slight angle like that would make any meaningful difference to anything. Sidedraught SU carbs are old, old, old school technology and as long as they're not vertical or upside down they should be fine. Surely if it made that much difference we'd all have problems when we braked or accelerated and the petrol sloshed forwards or backwards? Also the needle will always provide the same restriction in the jet whatever the height of fuel. If both needles are filling 50% of the area of the jets then the fuel height dosent matter. Fuel is drawn into the carb by capilliary action and it is the restriction of the needle in the jet that provides the metering of fuel. So at any slight angle it wont make a bit of difference. If I were you I'd be checking out other aspects of the carbs!