When looking at the fuel burn across the four Lycoming engined ( not injected ) aircraft that I own I would say that the engines all the have about 18-20 % less fuel consumption when leaned IAW the Lycoming instructions and that is slightly RoP.
As for fuel planning VFR I use TRIP FUEL + DIV FUEL + 45 MIN HOLD + 5%.
TRIP & DIV are calculated at the power setting to be used with the HOLD calculated at 55 % that equate to the best L/D speed then 5% of the whole burn.
If it is a long trip (over 4.5 hours) and there are a lot of airfields that could be used as an en-route alternate then I will reduce the 5% to 5% of DIV+45 MIN HOLD. As long as the fuel plot stays at what is expected then it is OK but if after two hours or so the fuel plot drops below the expected line you will have to pick one of your en-route alternates and land, this will ensure that you land with a (very) safe amount of fuel but it will require a lot of discipline to avoid the temptation to try to push on in the hope of the fuel plot coming back towards the line.............it usually won't !!